Work is draining my battery with no signal

I’ve noticed that my IPhone 13 is not lasting a day at work without needing charging and as it’s only a year old and all my previous phones lasted the day I checked what was using most battery. It’s the no mobile coverage. You can clearly see the two days I was at work and used my phone the least are the two days where my battery used the most (Thursday and Friday in case you can’t see) I’ve always known signal is poor there so connect to the WiFi when I walk in and my previous iPhones coped fine. I’ve had to bring my portable charger in and keep charging during the day which is destroying the overall battery health depleting the resale value so it’s going to get worse. Was going to treat myself to a new phone for my birthday but if the battery is going to drain like this lowering the value of my phones I’m not sure it’s worth it. Not sure who to get frustrated with? Phone or network?