Wi-Fi and Bluetooth

First try a Forced restart

Force restart iPhone – Apple Support (UK)

If no better do Settings, General, Transfer or reset iPhone, Reset, Reset network settings.

After that, If Wi-Fi will not connect 

·      Reset the router to factory settings (menu or paper clip)

·      On iphone Settings, General, Transfer iphone or reset, reset, reset network settings

·      If you have a VPN you are strongly recommended to uninstall it and its profile in Settings, General, VPN and device management 

Here's some more steps you can try for Wi-Fi connection issues that may be helpful if your phone will not connect to WiFi

If your iPhone, iPad or iPod touch won't connect to a Wi-Fi network – Apple Support (UK)

Finally...

Backup, restore to factory,  and   test, then restore from backup:- 

(If the phone needs attention you will have to do this anyway)

If you can import your media to Photos or iTunes in a computer and copy important documents to a cloud server it will give you some resilience in case the restore from backup fails, or you need to ditch the backup and start as new. 

(1)          How to back up your iPhone, iPad, and iPod touch - Apple Support

(2)          Restore your iPhone, iPad, or iPod to factory settings - Apple Support

(3)          Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If the problem remains then do step (2) again but set up as new without restoring any backup. You would need to rescue your data from wherever you safeguarded it but you will lose all message history.

 If that fixes it then it is probably a rogue app.

Otherwise arrange a repair

iPhone Repair - Official Apple Support