No Internet After Setting Ethernet Adapter to Static IP

Weird issue - neither Comcast Support nor I can figure out what the issue is.
Server is Windows11 pro with IIS installed.
Modem is in Bridge Mode (advanced).
There are 2 routers plugged into the modem.
Server is plugged into Router A.
Router B is working fine, and both wifi and hardwired devices (using DCHP) have  internet access.
Router A can see the server, and I have assigned it a static LAN address.
The issue is that the server only has internet connectivity in DCHP mode.
When I try to assign my Comcast static IP address to it, it cannot connect to the internet, though it is still visible on the LAN.
I've tried configurations using the gateway and subnet provided by Comcast, as well as the ones used by Router A. I've made sure that the port is forwarded on Router A, and that the port is open on the server.
I've tried turning off all the firewalls.
I've tried clearing the DNS cache. I've tried 2 different ethernet adapters, and 3 different ethernet cables.
I've rebooted the entire system countless times.
What am I doing wrong?

Help?