WireGuard problem with dual WAN, replies only come through the main link

Read @lurker888 posts (and also the linked threads) for explanation:

Follow those threads for the workarounds too.

The upcoming 7.21 (currently rc2) update to RouterOS will simplify the workaround because VRF will be supported, as an example: Wireguard VRF Issues - #19 by CGGXANNX.