Thanks for the update.
I too have relatives with access via Full Fibre Ltd but it is such a gamble to recommend as there is no way of knowing what the service is going to be like in a specific area. It is a pity the Trust Pilot reviews do not give their location so you know if a specific location is having problems.
The only thing I do need to ask is how do I check this as the IPv4 address on the router starts with 100.64 which seems to be a CGNAT address, but I might be wrong here. Is there a way to prove whether the IP address is static or CGNAT? It's caused us no issues so far so I've not looked into it further but it would be good to check it.
If the IP address is still in that range it is CGNAT.
Does anyone know if you need to restart the router or even the ONT for IP address changes to be picked up?