How to fix an iPhone from dropping cellular signal?

There’s nothing you can do yourself. It’s either your cellular carrier’s signal being unstable or there’s a hardware problem with your iPhone 7 Plus. Make an appointment with an Apple Store or Apple authorized service provider for examination and diagnostics if Apple even supports the iPhone 7 anymore. They may tell you the same thing as Bob Timmons did.

That 7 Plus is as much as almost 8 years old and it can't run anything close to newer iOS versions.

It's time.