WiFi drops off iPhone constantly

[PBR123]

I have a sim only contract and an iPhone 6 which works perfectly. However since using the sim last week every time I try to use the WiFi or tether to my iPad the signal drops off immediately making it unusable. It worked for about 2 days but hasn't worked since

has anyone had this issue and what can I do to resolve it? 
I've tried all of the usual things and it's definitely not the phone as I have used another sim in it and it worked perfectly 

if I can't resolve it I will have to cancel the contract.

any help gratefully recieved

[madasaf1sh]

@PBR123

Have you done a network settings reset from within the General Menu?

Also turn off WiFi assist in the menu as if the connection is slow it will default to Mobile Data...

WiFi and your sim card have no correlation.

I would say try these steps as well:
https://appleinsider.com/articles/19/01/11/how-to-your-iphone-repeatedly-dropping-wi-fi-network-conn...
The other option is to Factory reset your device after a backup to icloud.

[PBR123]

Thank you I will try that