Port forwarding issues AMG1312-T10B

I own an AMG1312-T10B router provided by my ISP. First, I succeeded port forwarding from 443 (external) to 8321 (internal) ports to 192.168.1.X, then, when I try to set a new rule, port forwarding from 3218 (external) to 3218 (internal) to the same IP (192.168.1.X) the page freezes with the login icon and then I lose access to the router, I've tried using a different port also. I don't know why this happens, I am following a tutorial in which they do the same configuration in another router and it works. 
Appreciate some help

#CPE_Sep