Hmm. I pulled my Beryl out of its case and had a go at this, from within my AC86 home lan [so, completely invalid for anything!!].
I used my Openvpn client .conf file within the GL-inet GUI, successfully made the connection…and was not able to reach my local network. So I deleted it, and followed the Luci steps I noted earlier, successfully made the connection, and was now able to reach my local network. 
So I’ve replicated your experience and a possible solution. You might try it…I’ll have to get out of my basement and get a cell signal to verify it works in a proper fashion, and if so we can move on to why the GUI is having an issue.