DX3301-T0 web interface timeout

Dear all,

I would like to request help with the web interface of DX3301-T0. When connecting from WiFi via local IP address, it works fine. When connecting from WiFi vial global (WAN) IP address, it is considerably slower. When connecting from my phone 4G connection via global IP address it is so slow that it times out and the interface is unusable. Addittionaly, my browser always reject the router's SSL certificates. Do you have any suggestions on how to:

-Connect with the router via my phone 4G in a tinely manner;

-Install trusted SSL certificates for web based login.

I got the router from my ISP in Slovenia, A1 Slovenija.

Thank you.