I have the same issue on mobile data, asked to sign into network via an android notificaton then get a fialure to load the ISP verification site due to "too many redirects"
just got this from online chat:
"we have service outage with bt mobile network due to this all customers are facing mobile data problem. We have already escalated this problem to network team, it will be fixed very soon"
to many redirects to this site - ukisppageverificationblocking.co.uk
the domain is owned by BT.