My experience with this issue is that changing mobile data to 3G only it keeps a stable (albeit very slow) connection. Once connected I can change it to 4G, works ok but then it looks like on re-negotiation or something like that (20min ish) it goes to no service again. Put the phone in flight mode, change back 3G and let it re-connect to the providers network and it works again. Leaving it on 3G, as mentioned above, it remains connected. Weird thing I find as well is that when I normally roam to O2, I now have either EE or 3 (in UK with Belgian Base number) Works for me on 14.0.1 and 14.2 beta [current iOS]

Small update on my experiences so far. Upgraded to 14.2beta3... same issue when roaming, still no service if on 4G, I force it on 3G, connects can change for a 20ish min to 4G and have to redo. Still not roaming to O2 network but EE, when on EE it seems to connect but unable to make/receive calls (even on 3G), I have to force it to 3 and unable to force to O2 or Vodafone to make/receive calls. Now back in Belgium it works ok on Base 4G, no no service, no need to force to 3G [Base sim] iPhone XS

3rd update, for me the issue only pops up when roaming. Spend a few days in Belgium so on my "local" provider, no issues what so ever using 4G. Now back in the UK, no connection on O2 or Vodafone, just does not want to connect. EE still same, connection but unable to receive/make calls, force to 3 and mobile data 3G works fine [unless 3 has no coverage in an area]. Still this does not solve the no service issue and should be a termp workaround. iphone XS iOS 14.2beta3 [because got a stable connection not sure of upgrading to beta4 unless I see someone that can confirm it fixes the issue]

Updated to 14.2 "official" release, still need the workaround of manual selecting the carrier and 3G only. At least this allows me to make and receive calls. Having said that the carrier I normal roam to [O2 UK] still does not allow me to connect and similar to the others I mentioned in my previous posts.... How hard is to put the code from a previous version, that work for that piece of hardware back into the new iOS. If hardware changed, use a hardware check during boot... iPhone 12 goto that section, below that section of code....