Take all DNS entries out of Network preferences.
You don't mention if your external IP is Static or DHCP. If it's DHCP and you have entered any DNS servers in the router remove those entries too. If it's Static use the DNS servers your ISP recommends in the router.
Also how do you have the routers configured since you're using two, do you just have them as APs or are they both being used as routers.
Also if you have the Firewall enabled on the Mac turn it off.
If it works after trying all of that, make changes one by one to your devices and test again.