I spent more than a week chasing config problems.

No offense intended... but it's like a dog chasing it's own tail-- you fix one with the router itself it creates another issue which creates 2 more with the firewall software on the PC's... it is not impossible. it
IS frustrating + time consuming.
Jot down changes you make to what... +when
see reactions
try another adjustment--
do the same.
This is why IT network techs for large companies ... are constantly busy.