Hi
I would welcome any suggestions to explain the issues I'm seeing.
It is complicated as this was set up for my Mum and I'm not close enough to visit.

New house no phone or internet line.
Bought Archer MR200 and Giff Gaff Sim. (Strong coverage as Giff Gaff Sim also using in mobile for calls).
Put SIM in fired up MR200.
Connected by wifi from Win10 laptop to MR200
Logged into control panel, changed SSID and password.
Laptop has great internet access.
Fire up Chromebook - again connects wirelessly - great speed.
Fire up Android Tablet - again connects wirelessly - great speed.
All good.
All machines work well for next approximately 36 hours.
I have now left with my laptop.
Text from Mum - no internet.
Get her to read everything to me.
All lights show wifi and internet.
No issues with wi-fi connection to router.
However no internet on tablet or chromebook (Router shows internet)
Usual rebooting of things.
Get internet for a few minutes intermittently then goes again - if left will come back on slowly for few mins but usually not enough to do anything.
Got her to connect to the _5G SSID but still the same.
So got her to hit factory reset button.
Came back up, obviously lost my SSID but used the default.
No issues wifi and internet steaming along on tablet and Chromebook on both normal and _5G links.
All seems great no issues.
24 hours later - grinds to a halt and no amount or factory or other resets do anything other than give brief flashes of internet.
All the time the router shows connected to internet.

At my wits end especially as having to troubleshoot via phone to Mum who is not the tech savvy :(

Any guidance welcome please.

Many thanks

Chris