UDP ports and firewalling

Same issue as described before, I can see data from my nodes on the gateway traffic console, but no ATAA activation.

I am using a Linksys EA7500 router since recently, and set the port forwarding to port 1883. The previous router (Buffalo) just worked with TTN. I no longer have the Buffalo router, and am trying to understand the reason why to does not work for TTN, everything else works fine.
The gateway is a Multitech conduit, updated to 1.4.3 and using the TTN packet forwarder.
My ISP provided a public IP address.

When checking the port using ping.eu, I find port 1883 closed, although opened at the router.
I can open and close port 8 and 443 using the router’s port forwarding setting, but port 1700 and 1883 remain closed, although opened on the router.

Am I onto something, or is this to be expected?