I have an OpenWRT router with one Ethernet WAN port, two Gbps LAN ports (one connected to a dumb 8-port Ethernet switch) and WiFi 5GHz onboard, yet no USB port. There is an Arch Linux ARM powered Pine64 ROCK64 SBC in the LAN, working 24/7, having one Gbps LAN port and multiple USB 2/3 ports. I also have a USB LTE modem. I need to set up the modem on the ROCK64 device to act as a WAN interface, with the OpenWRT router still doing routing, DHCP, WLAN etc. (probably still having its Ethernet WAN interface active and having MWAN on both WAN interfaces). Is it possible to do so?
Yes, it is possible, but you'll need to separate the rest of the lan from the Pine64, as the mwan3 will need to have a dedicated wanb interface there.
So it's either Pine be the sole WAN device or the multi-purpose server without WAN functionality? No way to set up two virtual interfaces (WAN and LAN) to share the only Ethernet port?
UPD. mwan is not a necessity (if it matters), the priority is setting up LTE as the only WAN.
That's correct, you can make vlans to create 2 subinterfaces and use on for lan and the other for wan.