Help request. Draytek 3900 halted all incoming connections. What else can I try? xpost from r/sysadmin

I've been hacker typing at this for 8 uninterrupted hours, give or take dinner at my desk. Anyways, I've got this client with said router, dual WAN, one line from spectrum and failover to at&t. Humming along just fine for a solid year, with maybe 10 hours downtime for maintenance and power outages before the building's backup power was installed.

All of a sudden, all incoming external connections except for the router's web config via https have ceased functioning on both WANs. Won't even accept my VPN connection(hosted on the router).

We have one host on DHCP reservation running one solitary service, requiring only one port, no other port redirection requirements, no upnp, no dmz. Everything else is outbound, approximately 300 devices on the network, no other connectivity problems for users. The host can be accessed flawlessly from the local network, as determined by a quick remote desktop session.

Power cycled everything, including the host device. Tried deleting and recreating the port redirection rule in one to one mode as it was. Several different firmware versions. Backed up router config, tried old known good config files. Tried different external ports. Tried different host ports. Tried different services on different devices(temporary setups). Tried DMZ for shih tzu giggles. Nothing. I wanted to blame it on the ISP, but it's the same on WAN2...doyyyyyy...

Neat trick, I can redirect any port of my choosing to the router's non standard external https port and connect fine. Just nothing gets PAST the router.

I guess I'm gonna have to leave my bed... Merrrrrrrrrrrrrrrrrrrrrrrrrrrrr.

All that's left, unless anyone has any genius ideas, is to begin physical process of elimination.

Thanks admins.

Update 1: packet monitor picks up my external requests on that port. It's just no longer passing them to the host... I've put in a ticket to Draytek. It's well within warranty.

Update 2: woke up to an open port... WTFFFFFFFFFFFFFFFFF... yay. Makes me think it really was the ISP. My tired brain is failing to draw any other conclusions.

Update 3: issue returned just as quickly as it left... idk