However I learned, that the carriers are using different frequencies in different locations
Yes, that is the case. Frequencies of 850 MHz and lower are primarily used in rural areas, with the higher frequency bands used in urban regions. Bands 4 and 66 are dual frequency bands with 1700 MHz freqs. used for uplink and the 2100 MHz freqs. used for simultaneous downlink (Verizon).
The following information may not be the most current or accurate as to the individual Bands’ role in transitioning into the 5G era. References to the 2 & 3G bands do not indicate whether or not they are being transitioned into the 4G VoLTE or 5G bandwidths.
Here is a breakdown of all the individual LTE bands that AT&T uses and their role:
• Band 2 (1900MHz frequency range): a core AT&T LTE band with 20x20MHz blocks in most markets.
• Band 4 (AWS-1700/2100MHz): this AT&T LTE band is used as a supplement for improved capacity and is usually deployed in small, 5x5MHz blocks.
• Band 66 (AWS-3-1700/2100MHz): AT&T LTE band 66 is a superset of band 4, meaning that it includes all of the band 4 blocks plus adds a few more. AT&T usually deploys this in 10x10 chunks, and you could commonly see it in the New York and New Jersey areas. It is actively being deployed.
• Band 5 (850MHz): this AT&T LTE band is used most commonly for 3G (HSPA+ ) connectivity, but some of it also goes toward LTE. AT&T owns a lot in this frequency range throughout the nation, and band 5 is sometimes used in areas where there is no band 12/17 coverage.
• Band 12/17 (700MHz): the backbone of AT&T's LTE network and it provides practically nationwide coverage.
• Band 14 (700MHz): AT&T has a nationwide license for band 14. The carrier acquired these bands from FirstNet and they will be used for a federally-funded public safety channel. These will only be deployed in states that opt-in to the FirstNet service.
• Band 29 (700MHz): this is a supplementary channel. AT&T purchased this from Qualcomm and it is mostly deployed in a 5x0 configuration, meaning that you get one small 5MHz block for download (in some limited places like the California coast and northeast you have 10x0 blocks). This band cannot be used for upload.
• Band 30 (WCS 2300MHz): another supplementary band for 4G LTE. AT&T has deployed chunks of 10x10 across the nation.
Here is a detailed breakdown of the 4G LTE Verizon bands in use:
• Band 2 (1900MHz): A Verizon band is actively transitioning from 2G/3G for use for LTE. It is currently a supplementary carrier that brings more capacity to the network and is commonly deployed in 10x10 chunks.
• Band 4 (1700/2100MHz): This Verizon band has solid amounts of these bands that it deploys in larger, 20x20MHz blocks in many markets.
• Band 66 (1700/2100MHz): this is a superset of band 4 (meaning that it has all the frequencies of band 4, plus a few additional blocks). It is usually deployed in small chunks and it is not available everywhere.
• Band 5 (850MHz): A Verizon band still in use for 2G/3G services in some markets, while for others, it is using this band for LTE. Verizon holds a lot of this spectrum nationwide and usually deploys it in 10x10 blocks.
• Band 13 (700MHz): A Verizon band that is the backbone of the Verizon Wireless 4G LTE network. Verizon has this rolled out to most markets across the nation, but since it is usually deployed in rather small 10x10 chunks, it could become congested fairly easily.
• Band 46: These are frequencies in the 5.9GHz spectrum that the carrier has started using as of 2020.
• Band 48: This is a CBRS band that is unlicensed yet but has been in use since September 2019.
And here is an overview of the T-Mobile bands:
• Band 2 (1900MHz frequency range): this is a band mostly used in rural areas, or where band 4 is not available. It has a higher reach, and it is widely used in the Northeast to provide 4G coverage to distant places. T-Mobile has deployed various chunks of spectrum, from smaller 5x5 blocks to larger and speedier 20x20 blocks. This band is also used for 2G and 3G.
• Band 4 (1700MHz/2100MHz): the backbone of T-Mobile's LTE network. This T-Mobile band is usually deployed in large 20x20MHz chunks in most markets, providing fast speed and a stable connection. Used for more densely populated areas.
• Band 5 (850MHz): Extremely limited use. Most band 5 coverage is offered by Verizon and AT&T. T-Mobile only operates LTE on this band around the Myrtle Beach, South Carolina area.
• Band 12 (700MHz): This is T-Mobile's "extended range LTE" band, used mostly as a complimentary band for coverage in rural and suburban areas, and it is similar to band 71 in function. It is supported on most phones.
• Band 66 (1700/2100MHz): an extension (superset) of band 4, this band is supported on devices since late 2016.
• Band 71 (600MHz): the big win of the FCC auction for T-Mobile, this band was previously used by UHF TV stations. T-Mobile band 71 is owned by the carrier all across the nation in big chunks and will be deploying it in the near future. Since this is a 600MHz range band, it will have wider coverage and will improve coverage inside buildings. It is supported only by newer phones like the 2018 series of iPhones.
It also may make a difference, if you have SIM card of a reseller (MVNO) or the cellular network operator.
Of this, I am not sure. I have checked many of the better-known MVNOs and it appears to me that the IMEI check function of each vendor routes through the main carrier. As such, my results have always been the same regardless of Carrier or MVNO status.
that devices are blocked even if there are compatible. It's possible, that the network carrier want's to have a license payment per device, at least for 5G.
Now, there’s a thought… Love it or hate it, that’s how most things work on this side of the pond.
A solution also could be, that we became an MVNO ourselves.
Please, pursue this option. It will certainly expose exactly what is happening here.