It is not clear (to me) what you are trying to do.
Let's call the router in the middle of your scheme "failover/balancing router" and the one in the bottom "client router".
Modem1 - ether port is on VLAN10?
failover/balancing router - ether1 port is on VLAN10?
Modem2 - ether port is on VLAN100?
failover/balancing router - ether2 port is on VLAN100?
failover/balancing router - ether3 port is on both VLAN10 and VLAN100?
But how exactly is the bridge on the failover/balancing router with ports ether1, 2 and 3 in it configured?
That is the part of the configuration that you believe relevant, while what you need to post is your FULL configuration. (FULL as COMPLETE, without removing anything from the output of a /export, only changing anonymizing possibly sensitive data). This way we will also know the exact model of the router and the RoS version you are running, that may be relevant.
Also, you posted a scheme of an only wired network, not using ether4 or ether5, and now you have these other ports and the mentioning of radios (I presume wi-fi links), please update the scheme to reflect your actual situation.
Consider how the more info you fail to provide or provide vaguely or incorrectly the more time will be needed to (hopefully) find a solution to your issue, compare with "I'm ill, doctor. Help!": https://jdebp.uk/FGA/problem-report-standard-litany.html