iPhone 8 Plus WiFi issue

1. Intermittent Wi-Fi connectivity:

• • Restart your Wi-Fi router and modem.
  • Check for nearby interference from other devices or networks.
  • Disable and re-enable Wi-Fi on your iPhone.
  • Reset network settings on your iPhone (Settings > General > Reset > Reset network settings).

1. Slow Wi-Fi speeds:

• • Restart your Wi-Fi router and modem.
  • Check for firmware updates for your router and modem.
  • Move your iPhone closer to the router or switch to a different Wi-Fi channel.
  • Disable and re-enable Wi-Fi on your iPhone.

1. Unable to connect to a specific Wi-Fi network:

• • Restart your iPhone and router.
  • Check the Wi-Fi network’s password and ensure it’s correct.
  • Forget the Wi-Fi network on your iPhone (Settings > Wi-Fi > [network name] > Forget This Network) and reconnect.
  • Reset network settings on your iPhone (Settings > General > Reset > Reset network settings).

Just to confirm, you've worked though all of the steps here?

If your iPhone or iPad won't connect to a Wi-Fi network - Apple Support

If so, then your iPhone may well have suffered a hardware failure. Apple will service it - they'll probably just replace it - for $349 in the US. Prices may be different in other countries. Take it to an Apple Store or authorized Apple service center for further diagnosis.

For me that old an iPhone wouldn't be worth paying to fix and I'd put the money toward a new, or at least newer iPhone. Up to you, though.

Regards.