Common causes of poor cellular reception?

Before answering your “why”, we need to ask “what”.

What country?

What model?

What carrier?

What signal?

What band?

What did you do before it gets bad?

Thanks Tom for your questions; my answers below.

Please Note: The issue is specific to this phone, most probably hardware; that's why I asked about hardware type causes.

Your questions:

-What country? - Australia.

-What model? - (My question starts "iPhone 8 plus") specifically A1864.

-What carrier? - Same with either Telstra or Optus.

-What signal? - Only phone network signal is bad, WiFi and Bluetooth work perfectly.

-What band? - I don't know; I'm sure it's not relevant, because a similar phone (A1897) works perfectly with both SIMs, so it's not related to carrier or band.

-What did you do before it gets bad? - Purchased it (used) online; seller sold it due to poor reception, I know nothing more of it's history.

(one of my hobbies is to fix broken things to give away)

So, is there a common (hardware) cause of lost network reception in an iPhone 8 plus?

At this point I'm discounting settings and software, because both phones have been factory reset and both since updated to version 15.2, no improvement.

@jlw42 Signal and band type may be relevant because we need to know what RF path is being used. However since you bought it second hand, asking here is pointless since it could be anything now, open it up to check if there are any signs of tampering.

Your comment suggests there might be different antennae for different bands; is that correct?

if so, how many are there, and which ones are used for which bands?

Not sure, but I think I'm on LTE.

Is it shown somewhere in settings?