@bigbluesky5151 If the only requirement is to manage the pcWRT router from the ER4 side, the easiest way is to set up port forwarding on the pcWRT for TCP port 80 and/or 443 (to the router itself). This is a security risk when the pcWRT is on the edge. But since you have the ER4 on the edge, this is not a problem.
Since the ER4 subnet sits on the WAN side of the pcWRT, you won’t be able to access the LAN side of the pcWRT even with static routing. Both NATting and the firewall prohibit direct access. The pcWRT firmware is built on the assumption that it functions as a router with NATting & firewall enabled (i.e., with the WAN side exposed to the Internet). Turning off NATting or firewall will bring unexpected behavior.
If you need to access the LAN side network of the pcWRT from the WAN side, the best solution would be to set up a VPN server on the pcWRT.