WAN and LAN through the same cable ?

I have OmniTIK 5 PoE ac and ZTE MC889A 5G modem on the roof. There is only 1 LAN cable. Can I connect modem to Omnitik, Omnitik goes to main router, VLAN my way from modem to main router and then use it as WAN connection for every other device in the network (including that Omnitik on the roof) ?
I haven’t had such case, I just wanted proper input for configuration.
Thanks

Sure, why not ?
You simply need to clearly identify what traffic goes over which VLAN and then it’s a normal VLAN setup.

Absolutely yes you can. Perfectly viable configuration.