There's a loose red wire at your server
Your setup looks a bit messy.
How about this:
Make one of the 750's ports a WAN port.
Connect your weatherproof AP dish to that WAN port.
Assign a (
the) static address to that WAN port (or create a DHCP client on that interface)
Setup NAT, firewall, routing and DHCP server on the 750 (which shouldn't be more than 10s just utilizing a mouse).
Follow nepal's advice to disable DHCP on that WRT54G router - I guess you accidentially connected your network to the router's WAN port and now it does what it is designed for: protect the network behind the WAN.
Good luck!
-Chris