"Easiest" solution would be to configure the WiFi first, connect to the device via WiFi and then set-up the port as WAN. You should probably consider allowing SSH access from the WAN side, in case in the future you have issue with WiFi and you want to access the device using a cable via the port that's now WAN. You can also allow web access (to be able to access the GUI) from the WAN side, but this is not recommended if your device is the first firewall between your network and the outside world.
The link below talks about the subject, but you can easily achieve the task (allowing SSH access from WAN and changing the port to WAN) form the GUI if you prefer. Note that you would want to set the protocol to PPPOE rather than DHCP.
There might be another approach using VLANs, but that will involve using an extra switch and I'm not even sure it would work, and wouldn't be very practical if it works!
The tricky thing though would be to access the device via a cable of something wrong goes with the WiFi. I haven't thought of it in my earlier reply, but with the WAN port configured as PPPoE, accessing the device with have to be via the upstream router-modem and internet, so not the best option for contingency.
Another option would be to add another WAN interface with Static IP (on the same interface as the PPPoE except that it's Static IP), on a subnet different than your LAN of course. So that interface should just be there doing nothing until you need to access the device by cable, so you connect it directly to your PC, set your PC NIC IP in the same subnet as that WAN interface and that should give you access (provided you allowed port 22 from WAN side as explained earlier).
I expect that to work but never tried it, so you may ask a second opinion.
i don't have ssh connection via lan anymore. I have ssh only via wifi. So i thought changing something on lan and i messed everything. . Now i reinstalled everything. now happily using my router. But now i stuck in DnsCrypt-proxy setup to avoid DNS-hijacking issues, but the luci-app-dnscrypt gui is using very old resolvers. It doesn't have CloudFlare 220.127.116.11 dns.
Hi, after configuring and connecting to isp router as wan. Now i dont have any connection to ssh via port anymore. I can ssh only via wifi. Can you tell me how to get connection to ssh via lan port ?? plzzz