I have the RT-AC5300 with merlin installed and I was trying to connect my HP printer to it. An IP address was able to be generated and received a ping but after it was finishing configuring it tells me I have a firewall blocking the communication and to make sure to open port 427 UDP. I go into the port forwarding settings under WAN and add port 427 for the printer's IP but it still says it's blocked. I look at the firewall settings in the router and I see you can add an inbound so when I enter in the printer's IP and the port it tells me the IP isn't valid. I'm not sure how to allow the port if I've done what I believe is correct. The only other thing I can think of is disabling the firewall on the router. I haven't tried on the stock firmware because soon as I got the router I quickly installed merlin.