How personal is a home IP address?
When I explained that to them and asked how to unblock it, they introduced me to the unexpected concept of a dynamic IP address, which might previously have been allocated to someone who misused it on that site and triggered the block.
Since I don't use mobile internet at all, I'd assumed that my IP address was static - but it turned out to be dynamic, instead.
Hunting around in search of clarification, I found this useful guide to finding out whether one's IP address is static or dynamic. (It works, although getting into Command Prompt entailed some lateral thinking.)
https://www.techbout.com/check-ip-static-dynamic-4135/