I have a Mofi 3500-3GN ver. 2 rev. 2 (which the company ships with OpenWrt-based but customized firmware, their version "9.2 std" somewhat based on Barrier Breaker r36346, kernel 3.3.8).
When upgrading from the company-provided firmware to OpenWrt 18.06.1, I thought the WAN stopped working, but in fact I needed to move my WAN cable to the first physical LAN port and then it behaves fairly normally.
(I upgraded the firmware with the following two .bin files)
The RT3050 chip has one CPU eth port which passes through an internal 6 port switch. The port numbers on the case may not match the switch ports. It is the manufacturer's choice how to wire them out.
Standard advice is to log in via wifi while changing the Ethernet configuration. Use swconfig dev switch0 show to see which port is actually active as you plug an Ethernet cable into each port.
If the LAN and WAN ports belong to the same switch then you can change the config in VLAN configuration. But as @lleachii suggested, it will be much better if you reset to defaults. This is because between v14 and v18 a lot of changes have happened.
If you mean reset to factory defaults in the web GUI, (or umount /overlay && firstboot from command line), then yes I am resetting on each firmware that I've tried (and not selecting "keep settings" anyway).
Okay I have the 4-pin onboard serial hooked up anyway so I'm doing it that way. With nothing connected:
root@OpenWrt:/etc/config# swconfig dev switch0 show |grep link
link: port:0 link:down
link: port:1 link:down
link: port:2 link:down
link: port:3 link:down
link: port:4 link:down
link: port:5 link:down
link: port:6 link:up speed:1000baseT full-duplex
So I suppose port 6 is the internal link to the wifi radio. By connecting them one by one, I note a dmesg such as:
[ 928.829080] rt3050-esw 10110000.esw: link changed 0x01
Where hex column means from the dmesg "link changed 0x01" type message, port means from the swconfig command you suggested, and label is the physical writing on the router case.