I am a developer. I have my own roms for Oneplus 6/6T/8/8Pro/9/9 Pro. In addition, all my roms can be used with locked bootloader. Why would I use any other rom as my daily driver?!
Having said that, AT&T 4g/5g issues have NOTHING to do with the rom you use. They want you to buy their own branded devices, and as far as 4G and 5G, they restrict any other device.