I do not see any reason why it would not work together with the MOXA. You would simply VPN to the WR31 and access the MOXA as normal. Alternatively you could port forward but make sure you have strict firewall policies because you don't want things like this hacked.
Don't forget that the WR31 has 1 x Analogue input (0-10V, 4-20mA) and 2 x DIOs and the serial port is an RS232,422,485 port that can do Modbus/RTU and Modbus/ASCII to Modbus/TCP/IP. So you may not even need the MOXA for some sites.