The problem is solved, just add a routing mark on the rb750 route
chain=prerouting action=mark-routing new-routing-mark=main passthrough=no src-address-list=local-LAN
in-interface=bridge2-LAN
But in this case, you can see that the client cannot aggregate the sum of the bandwidth of the 3 ISPs, and can only connect to the Internet with a single line. Make a common mark, and then log in to the router with the mobile phone and switch the mangle statement.
Thank you for guiding me to solve the problem.