Peplink devices need named WAN interfaces. They come with physical WAN interfaces and then you can use virtual WANs to add more with VLANs (via a switch like you suggest).
A BR1 Pro comes with, 4 WANs, Cellular, Wired and 2 x WiFI (2.4 and 5Ghz) WANs. Then as its a primecare device (annual license first year free) it also has a single included virtual WAN (WAN via VLAN). To give a total of 5 supported WAN connections.
That virtual WAN is part of the primecare license and primecare needs renewal from year 2 to maintain it along with management via IC2, hardware warranty, and SpeedFusion Licensing.
You can also buy up to two more vWAN licenses (one time cost) to bring the WAN count to 7 for the BR1 Pro 5G.