I have brand new Vigor 2926ac installed at a remote site and experiencing strange issues accessing it remotely over WAN (management from LAN side works fine). I can ping WAN IP from outside with no issues, there is no packet loss. However, web-gui doesn't load reliably over WAN. Sometimes after some time it loads fully or in part, but eventually times out after entering login credentials. I tried HTTP and HTTPS, also changing default management ports, this didn't make any difference. I also tried installing a basic TP-Link router on the same connection and could access its web-gui over WAN with no issues. Any ideas what could be causing this?

Speaking from experience all the DrayTek’s I’ve ever owned have had similar issues. Scheduled reboots help or open CLI access via SSH and then reboot that way normally they will bounce back up for a couple of days.