You mean you have replaced the complete socket assembly, connecting the new one to the bare wires that arrive at it through the wall?
If so, the first suspect is that reconnection. What were the attenuation figures previously? We need to be able to compare them.
Incidentally, over 100 ISPs could supply your MIL with broadband. It is basically provided by BT Wholesale. BT Consumer (the retail ISP) simply sell that service with some addons, like all the others. It's only Sky, TalkTalk and its resellers, and Vodafone that can't supply.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
My broadband basic info/help site - www.robertos.me.uk.
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Connection - AAISP Home::1 80/20. Sync 65258/14193Kbps @ 600m. BQMs - IPv4