This indicates to me that you access the device remotely, and therefore you can lock yourself out if you make a mistake.
So first, whatever change you do should be done using safe mode (Ctrl-X in command line toggles safe mode for changes done in the same command line window; a button in WebFig and/or Winbox does the same for changes done using that GUI, but you can use safe mode only on one interface at a time).
Second, the way you use the failover between your 3 WANs so far is not sufficient to access it remotely via WAN. There are two possibilities how to deal with this:
- the simpler to configure one is to configure a static route towards the IP from which you’ll be configuring into the default routing table (i.e. without a routing-mark) and also one per each routing-mark you’ll be using, all via the same WAN through which you’ll be accessing the device remotely,
- a more complex but more useful one is not to assign the routing-mark directly but to use connection-mark as a base for it, which allows you to let any connection from outside be answered via the same WAN interface through which it came in - see details here (start reading that post from the last paragraph which gives you the context otherwise scattered across the topic)