there is 2 possible methods:
1. as Fewi mentioned, you need to have own AS and IP block
2. to put mikrotik router in datacenter with good connection and from your local router to establish 2 separate tunneled connection to it, one trough ISP1 and other trough ISP2 and to load-balance traffic over them.
In the 2nd way your clients Internet connection will exit from IP address of mikrotik router in the datacenter .
My second example is not best practice but it will work
and will be expansive because u will pay for server in datacenter
Better pay for high speed connection from one ISP