An interesting experience with 1.8… My Asus router has a VPN server function. I set up the server a few years ago (with a connecting client on my iPhone) when I was traveling internationally to get USA entertainment while on the road. Yesterday, while traveling, I connected to the VPN on the home server and started Roon 1.8 on my iPhone – it connected! I was able to play FLAC 44.1/16 files without problems on a wifi and an LTE phone connection. I tried several times in the past with 1.7 and never got a connection. Is this a new capability??
No - glad it’s working for you now. 
When I VPN into my network and fire up Roon (only works if the device I’m on remotely is on Wifi - not on mobile data) I can access my core and search and start to play. After about 5-10 seconds - the track craps out and it skips to the next track. Same on 1.7 as 1.8. By the way - Plex sitting on the same computer as my roon core can stream multiple FLACs to me and others via their mobile app whether I’m on LTE or Wifi - and that’s with keeping the flacs as flacs (not compressing to a lesser bit rate).
Yes this is my experience too running a VPN on an Asus router with the latest Merlin firmware. Works very well, no problems with skipping etc for me.