No service on iPhone 13 Pro Max

Hello, I have an iPhone 13 Pro Max currently running on iOS 18.2. It has been working without any problems until last two weeks. For some reason, the phone lost the ability to connect to mobile networks which makes it unusable. To be specific, it works only sometimes and without any problems as it worked before, but once it happens, the only way to fix it is to wait until it starts working again by itself. The phone just drops into searching and then shows 'no service.' I have also noticed that 5GHz Wi-Fi is not detected during that time, and in settings, there is 'mobile service issue detected.' The intervals between working and not working are completely random. Sometimes it fixed itself right away, sometimes it takes hours.

I read a few articles about it on the internet and saw that it happens due to the back glass getting destroyed, the phone being dropped, etc. Yet I have taken good care of the device – I was using a phone case all the time, it was never water damaged or opened internally, nor was it dropped hard. The phone is in perfect condition.

I have tried restarts, airplane mode, reset network settings, recovery restore, DFU restore, using only eSIM or only a physical SIM, etc. Nothing worked.

What is weird is that when I tried the diagnostics mode by holding volume buttons and connecting a cable, nothing was detected. I did not get any errors even on the RF module.

I am trying to figure out whether it is a hardware or software issue and am quietly praying it is only a temporary problem with software because board repair is really expensive.

Did anyone encounter a problem like this?

Thank you so much for your help and have a nice day.