Nighthawk R7000 can't see servers

Problem: 

• I can host a server (Arma 3) and people on other ISPs can join.
• I can see servers hosted by others on other ISPs.
• I can not see/join one friend's server. He is on the same ISP. 
• He can not see/join my server either.
• He can also host and let people on other ISPs join.
• We (I and friend on same ISP) have forwarded all relevant ports, and allowed in firewall.
• We are both using Nighthawk R7000 router
• Also, I can not join a dedicated Arma 3 server on G-Portal established as an attempt to solve issue. I can join other dedicated servers for other games though. (Not sure about friend's status on this, but my guess is he won't be able to get a response from same dedicated server)

So, common between the two users with issues:

• Same ISP, same router.
• Can host but not join Arma 3 servers (ports 2302-2306 primarily used (?))

Appreciate any help to further problem solve.