No, default firewall rules won’t protect if a new pppoe WAN interface is added afterwards.
To be protected by the default firewall, go to Interfaces > Interface List and add the Bell pppoe interface to the WAN list.
If you were using Quickset, then I guess it should have already added the pppoe interface to the WAN list, make sure is there.
That is an interesting solution to the HHBell (all in one bell fiber modem/router). I actually refused the Bell modem/router from the bell technician and got there business modem instead (none of the home crap added).
Is there a downside to using PPPoE in this way???
Stock setup would be straight ethernet IP from the modem router like if it was a cable modem (dynamic) or is it a static cable type setup?
PPPoE I have no experience with so don’t understand what is being done here.
This is false information. Default configuration for quite some blocks access on interfaces that are not in either LAN or WAN interface lists.
mrz, you may want to check the MT Training Program if a ‘certified’ trainer is providing advice so erroneous! 
I stand corrected… absolutely right on recent ROS releases.
But may not be the situation always, even if the running ROS version is a recent one when default input filter rule was changed from blocking input on WAN to
filter add chain=input action=drop in-interface-list=!LAN comment="defconf: drop all not coming from LAN"
in the default config… e.g: initial ROS version installed not having this default configuration, that gets upgraded afterwards to a recent ROS version (won’t upgrade firewall rules).
Yes, a corner case, (that have found countless times and keep facing still) but this, taking into account the (old) router model of the OP was enough to lead me into that incorrect statement.
mrz, you may want to check the MT Training Program if a ‘certified’ trainer is providing advice so erroneous! >
Well, my statement may have been not 100% correct, specially to generalize without elaborating like I did, will self-flagellate accordingly.
But on the contrary, my advice was 200% solid if you take into account the “small detail” of the mikrotik router/Bell router models age OP is using. Check interface lists.