I've had my ASUS DSL-AC68U working just fine with the Vivid 100 service for a few weeks now. I wanted to change it to use a VPN service - PureVPN is the one I've tried. But I have no joy when I turn PPTP encryption on. It works fine if I leave the PPTP option to 'Auto' but changing it to MPPE 40 or MPPE 128 causes all web sites to become inaccessible. If I use LT2P it doesn't work at all.
I can't find out if PPTP is encrypted in any way by default (anyone?) and so far PureVPN seem unable to help. They suggested the router doesn't support encryption but it is supposed to, even according to their own instructions.
I've checked the SuperHub - it's in modem mode so there aren't any adjustments to make there. Also, if I install the PureVPN client software on my Windows 10 machine (without adding VPN settings to the router), that appears to work too.