So I have an update here for the progress (?) made so far according to vic0239's suggestions per my interpretation (which cries for improvement to be honest since I had to translate RouterOS into EdgeOS - lol), which doesn't look good. Here's some screenshots showing before and after the modifications made.
As things stand my ER-X is on 192.168.1.1, mask 255.255.255.0, whereas the ZyXEL sits on 192.168.6.254, mask 255.255.255.0.
First off for eth0 which is my WAN on the ER-X I manually added 192.168.6.2/24 into the respective field and it appeared on the interfaces and routing tables. However upon creating either an (a) a static route type interface to 192.168.6.0/24 for eth0 interface (WAN) or (b) a static route type Gateway to 192.168.6.0/24 to next hop address 192.168.6.254 (ZyXEL IP) I still can't access the ZyXEL. In both cases accessing 192.168.6.2 lands me to the ER-X login page.
On a sidenote for the time being I also have an extra router as mentioned above setup on 192.168.6.1 serving wifi and VoIP with a DHCP server on and a pool of 192.168.6.100-110 for the wifi clients.
I'm now officially back to square one, please do let me know how I might go ahead and alter this setup and do bear with me as I'm way over my current skillset with this one