What is the easiest way to route youtube streaming video over WAN1 and all other traffic over cellular?
Create enforced WAN1 outbound policy rules with source set to any and destinations with the following domains:
Create another enforced rule for cellular interface with source, destination set to any and place it on top on HTTPS_Persistence rule. For all new created rules set protocol to any. With these rules you should be able to route youtube through WAN1 and the remaining traffic through cellular interface.