Nighthawk X6 800: a particular app doesn't work on wifi

Hi,

I have a weird experience with 1 particular app, called Line (very common one). I can't connect to internet eg receive new messages.

First i thought it was app related, cause I installed a new OS on my phone. However my old phone has the same issue.

On both phones I can access multiple internet related apps without any issue. 

I don't think I have any services blocked or ports in my Router, nor running Armor software. When i connect to a different wifi network, the app works without any problem.

To eliminate issues I restored my router back to factory setting, however the problem still persist.

It look likes my router blocks all network traffic from this app, but no idea how and why.

Any suggestions what I can try else? Firmware is up to date.

thx