Poor internet quality

[#1]

I am having trouble with my phone which is disconnecting from WiFi, and reverting to mobile data, despite being very close to the WiFI hub or repeater, with full strength. I have rebooted the router and repeater, and the phone (android), and the problem doesn't appear to happen away from home. 

Could the WiFi being or poor quality because the internet connection is poor?

We have had a lot of line problems in the past when we had a landline.

Any ideas pls?

[#2]

Settings to discourage mobiles from dropping Wi-Fi and reverting to the GSM mobile data.

For iPhones...

Disable "Wi-Fi Assist"    Ref:https://support.apple.com/en-us/102228

For Android phones...

Disable "Switch to mobile data"

Settings >> Connections >> Wi-Fi >> ... ( 3 dot menu ) >> "Switch to mobile data"  ( set to OFF )

[#3]

@skck 

What PN service are you on - SOGEA (FTTC without the phone), Full Fibre or a legacy account with both phone and internet?

Brian