Hi @aquablue,
Create enforced WAN1 outbound policy rules with source set to any and destinations with the following domains:
youtube.com
www.youtube.com
m.youtube.com
ytimg.com
s.ytimg.com
ytimg.I.google.com
youtube.I.google.com
i.google.com
googlevideo.com
youtu.be
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.