Setting up remote access to the network from Mikrotik when the gateway is an operator .. How to do it right?

Question

Mikrotik

Good day everyone. There is an operator modem, which is a gateway to the network. You can't change any settings on it. After this modem there is a router RB750. After RB750 access point RB912, subscriber stations RB SXT Lite5 and subscriber router RB hAP mini are connected to it. On the RB750, the operator's network was thrown into VLAN 100, the management network into VLAN 200 with its own subnet and, accordingly, DHCP. The access point and the subscriber station are configured in the bridge and pass everything through unchanged. On the subscriber router, the operator's network forks and is given to the subscriber. Naturally, the control of the subscriber is not given and VLAN200 hangs on the router just for control. At home, I raised an L2TP server, on the RB750 serviced network, respectively, of the client. In order to get to all microtics, you need to register a gateway on them (in my case RB750), however, the default is busy for the carrier network. Tried to add a gateway with dst. adress and RB750 as a gateway did not help. I understand that something else needs to be done, but I do not understand what exactly. How can I make my idea work?
PS If you default RB750, then external control works with a bang. But of course the Internet does not work for clients.

Answer 1

https://wiki.mikrotik.com/wiki/Policy_Base_Routing

Answer 2

Good day everyone. The solution was found. Romon helped.