downgrade iphone ios on 7plus

The latest IOS15 for my 7plus has resulted my phone signal to one bar and no service. It wasn't like this before. I have replaced the SIM card and reset the phone. At first I thought my phone needs repair. As I went to a repair shop , the phone signal were strong as they have a signal tower nearby. I believed the new IOS 15 causes the signal receiving range reduced so they want me to buy a new 5 G phone. I wanted to downgrade to IOS 14.8 but it is unsigned and it said: This firmware is not signed. This means you cannot restore to it in iTunes.

Please advise. I am using a Window 10 PC. Thanks