that internetipv6 thing is probably only in Orange - i do not see any info about it on PLAY as a provider.I'm on Orange PL and had the same problem, btw this problem is on every ROM on Android 11. The solution is to change the default APN from "internet" to "internetipv6" and also change the protocol to IPv6. How do you change on your network? I have no idea, for a start try changing the protocol to IPv6.
right now i'm on xiaomi.eu rom, LTE wrorks and in network settings i see as APN type just "internet" (nothing about ipv6) and APNs ptocol is set at "ipv4/ipv6" - all that i get automatically after a clean flash of LOS.
today i've try to flash from sd card (i ask myself - why not?), ressults are the same - i do not have mobile data on a11 roms