I have an E3276 and a 823DRU running OpenWRT, they've been rock solid.
One reason for the connection going down might be an underpowered USB port.
I don't know were is the problem, the interesting, that openvpn reconnects if there is some problem, but the internet to lan client no, ant i am always able to connect to the router, and lan client, also i am tired of this issue, and tired of padavan, it's good, but there is some issues, also it's at the moment already not updating, thats why we should forget it for the future, as the creator of this PADAVAN firmware, starter to work at KEENETIC, and now KEENETIC OS is on of the stable an one of the best firmware with nice interface OS, but unfortunately this is not open source, and you can use just with KEENETIC routers. that's why decided to make new project router, as cheap as i can also it should be small as it can be, and that's why i selected this small router GL-MIFI (also other reason was internal modem, to avoid disconnections and so on:) i hope it will work good, without any problem.