You can narrow down ether its MTU with a simple ping. If ping works but other traffic does not its possibly MTU. If ping fails its not MTU.
1500 is the native MTU on the internet. Typically people using VPN’s or PPPoE will have a reduced MTU unless this has been over-provisioned by the ISP, for example 1508.
But connections via IPoE (ie: DHCP assignment) or PTP-static (typically businesses) services will be 1500.
After reading this I suspect whatever DDoS mitigations have been put in place have been over-zealous.
If someone has a contact at Canonical to get our IP’s unblocked (if this is whats happened) please send me a PM.
If it was me; I’d likely ask in the Launchpad room on Matrix (even IRC though there will tell you they’ve moved to matrix anyway, ie. it’ll list https://matrix.to/#/#launchpad:ubuntu.com )
You can file a ticket (https://rt.ubuntu.com/) but I’d still try and use the other methods first (for myself, that would be a fallback option).