@Keneda Officially (according Fairphone Company) it works on T-Mobile:
https://support.fairphone.com/hc/en-us/articles/16674972235537-Murena-Fairphone-4-with-e-OS-in-the-USA
Excerpt below:
*Fairphone 4 with /e/OS will only work with T-Mobile in the US, as it is not optimized for Verizon and AT&T. This means that as long as there's T-mobile coverage, it'll work.
After the 3G network is shut down, the Fairphone 4 will continue working on the 4G network. It also supports VoLTE (Voice over LTE, a.k.a. 4G calling) and VoWi-Fi (Voice over Wi-Fi, a.k.a. Wi-Fi calling) on the T-mobile network in the US.*
However, I did travel to North America in August 2023, and my FP4 worked also perfectly well on AT&T network (4G, 5G and VoLTE). I didn't test Verizon as I remained in New Hampshire and Vermont states.
It worked also perfectly well on Rogers and Telus carriers in the nearby province of Quebec (Canada), even on 5G frequency.
Phone calls and cellular data were used without the slightest hitch.
For this travel I used /e/OS on the device. I was Roaming using my own sim card (Salt. Switzerland).
So I guess, when VoLTE and 5G support will soon come in Ubuntu Touch on the Fairphone 4 and 5, it should work fine as well on those networks.