100% Same issue here. I have 3 IOS devices in my home; 2 iPhone 8 Plus (Me & My Father) and an iPhone X (Mother).
Ever since updating to IOS 14 on my phone (iPhone 8 plus) and my mother's phone (iPhone x), both of us receive low to no signal. The status bar will show full reception until you try to make a call or connect to cellular data. The calls will continually drop out or fail. Text messages do not always send and the 4g connection will always time out. The only solution I have found is Toggeling Airplay mode on and off and then trying to make a call or connecting to cellular. However, this solution only lasts for about 1min and then the signal drops out again and the phone will either display SOS or No Service.
I've had this issue since late November 2020. Initially, I assumed it was my carrier (Optus at the time), so I switched to Vodafone. However, the same issue occurred. Then once again I switched my provider to Catch, But again the exact same issue. After about 1 week with a new carrier, the issue starts again, which is ridiculous. The apple technicians at the Apple store don't know what the issue is and try and dodge it at all costs; saying that it must be a hardware issue, which I find difficult to believe - I mean two phones, having the exact same issue after an update to a new IOS surely sound more or less like an OS issue.
My father on the other had A iPhone 8 Plus running ios 13.7, and the network runs perfect!!, I even switch my sim with his phone and the network works fine. When I put his sim in my phone (Running IOS14.4) the same issue with drop calls and reception issues happen again.
I have tried all the troubleshooting techniques, but nothing works. I have to do the 'reset network' almost daily, but still, nothing happens. As mentioned before the only solution I have found that works reliability (albeit only for like 1-5mins) is the Airplay mode technique.
I've heard that some people are suffering from this issue for over 5months, which is ridiculous. I hope apple provides a patch update soon.