RB750GL behind ISP's router

Hello,

I have three WAN links at the moment. Two of them are via cable company modems. Third one is provided via Huawei DSL router. I use first two for two VLANs. I want to use DSL as failover in case cable modem internet isn’t working. At the moment, internet via Huawei router isn’t working. I use dhcp client for all three WANs, but for some reason, I can’t even ping google via DSL interface. This third interface gets private IP from Huawei router (doesn’t work in bridge mode). If I attach computer in the same port on Huawei router, it gets the same private IP address and internet works. What am I missing?

This is the current setup:

WAN1 (interface port 1): cable modem > mikrotik (public IP) > VLAN1
WAN2 (interface port 2): cable modem > mikrotik (public IP) > VLAN2
WAN3 (interface port 3): DSL router > mikrotik (192.168.12.2)

When I’m using ping tool in Mikrotik, I’m getting timeout when pinging via DSL interface. What can be the cause of this?