Two causes, as well as it being bedtime
Primarily, you replied to me. Your post as now explained was surely a reply to the OP. As it was, because it was a reply to me and yes I was tired, I subconsciously equated your name, Michael, with the OP Mike. Your post being obviously of no possible interest to me adding to the confusion.
Secondarily doesn't the phone wifi hotspot also involve almost certain costly data usage?
As you say, I was in effect replying to the OP, and it remains true that his connection appears to have a problem that he needs his ISP to sort out. I suspect it is something directly controlled by the ISP, such as traffic management. Perhaps incorrectly applied in this case.
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.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. Sync 59546/15321kbps @ 600m. - BQM