Without having access to a known good connection (or known good setup) you can't 100%, but if you can take packet captures at the handoff to the isp equipment and the far side doesn't receive them, that's a pretty clear indicator to me. At the very least it would be enough for me to reach out to them and ask what's up.

Run the ooni probe and see what it reports.

https://ooni.org/install/

The best you can do, as far as I know, is run wireshark on both ends. You'll watch the packet leave your network and never arrive on the other end. This doesn't prove it's your ISP but it will prove it's not your fault.