Connect OpenWrt subnet to RT2600AC Secondary Interface

Hello everyone,

I am new to OpenWrt.
I would like to connect two subnets (I attach the network graph to make the idea).

Each of them goes out to the internet with its own modem.

For infrastructure reasons, I can only use one gigabit network cable to connect them (the one highlighted in the graph).

I would like to be able to use the Mikrotik's subnet as a failover of that of the RT2600ac, in addition to using the NAS at gigabit speed.

Currently, the cable from the RT2600ac's secondary interface connects to one of the Mikrotik's LAN ports (OpenWrt 21.02).

RT2600ac connects to the OpenWrt LAN with static ip

OpenWrt is configured in default mode (except for the LAN subnet which is
lan1 (WAN)
lan2, lan3, lan4, lan5 (LAN br-lan)

Unfortunately, the transfer speed between any device in the Synology subnet with the NAS is extremely slow (26 Mb/s).

There are the routing or configuration problems, which I cannot solve.