Okay , I think you have to create two static routes on the Peplink :
one : for destination network : 192.168.100.1 MASK 255.255.255.0 and Gateway 192.168.100.1 for the routing between Peplink <==> Mikrotik <==> Office Network
two : for destination network : 10.90.90.1 MASK 255.255.255.X and Gateway 10.90.90.1 for routing between Peplink <==> Mikrotik <==> Service Tower
Also Mikrotik should be configured properly to route packets between Peplink and ether(s).
let’s try it and let me know the results.