@RTouris 

If I understand correctly your AX55 WAN IP address is 192.168.1.50 and AX55 LAN IP address is 192.168.5.1.

If that's correct, there's no way to browse the web management interface of AX55 using it's LAN IP address from the 192.168.1.0/24 subnet.

That's because of the NAT that AX55 does. Same goes for the ping respond from its LAN IP address.

This is a home router and that's how it works - check this story (Basic Level of protection - NAT).

  @RTouris My understanding is that it **SHOULD** work in the presence of a static route (which i already setup for all the corresponding subnets) in the form of Destination 192.168.5.0/24, next hop 192.168.1.50 and interface switch0 on the ER-X. Out of all the routers involved in this config the AX-55 is the ONLY one that is inaccessible in it's LAN IP (while in the intraWAN), hence my legit failure to understand why this is so. 

  @RTouris 

This would work in a business grade router on which you can set its ethernet ports IP addresses to be in a different subnets.

AX55 is a home grade router with an integrated ethernet switch and HW/SW limitations will prevent this from working.- check this article - section "Issues and limitations".

  @terziyski 

I appreciate your interest in this however this doesn't explain why all the rest of the SOHO equipment behaves differently or why two very old TP-Link modem routers also appear to support this, namely the TD-W8980 and the TD-W9980...