This may be a sizable leap in debugging, but for strange networking issues, I’ll usually start Wireshark and monitor whatever traffic is coming from the ISP’s equipment, looking for clues. A really nice clue would be something like VLAN tagged traffic, which would indicate the ISP requires a certain VLAN ID. Or perhaps you could see if your DHCP requests are being answered or not.
I do recognize that this sort of network sleuthing is as much art as it is science, so your mileage will vary.