@2525rentcar 

Try making the PC2 IP (10_168_100_100) the DMZ IP for the ER605.

  @2525rentcar 

I should probably have asked why that particular topology to make sure the router will do what you want it to.

  @2525rentcar 

I don't understand the context of 'depletion'.

Given your requirements, there is no reason for the ER605 at all.  If you are setting up a homelab to learn about networking, then you need to understand that the ER605 is not a classical router like say a Cisco is.  It always operates in NAT mode, so to the network above it, it looks like a host, not a gateway.  If there are hosts below the ER605 that need to be reachable, then the only way is to port forward every port that needs to be reachable (or DMZ which is the ultimate forward all ports option).  This is what makes PC2 appear as if its IP is 10.168.0.218 and allows it to communicate bidirectionally with PC1.  Said another way, there is no path where 10.168.0.100 can talk directly to 10.168.100.100 through any TPlink router.

  @2525rentcar 

I think this is what you want to do?

Create two LAN subnets on the ER605, map them to two different LAN ports as below.

That will allow PC<->PC communications as well as internet access for both.

It will NOT allow Internet->PC access because the WAN1 port has a private (NAT'd) address (you need a public from the modem if that is to work and port forwards)

  @2525rentcar 

It is difficult to answer your question. You did not indicate what device that routing table is from? Is it from PC2? It does not look like one from a Windows machine. The first row in that table seems to be a route to the default gateway for Internet. However, there is this “128.0.0.0” subnet mask there. The IP range covered by that route is 0.0.0.0 - 127.255.255.255 so there is a route to some but not all Internet IP addresses. BTW, you will not be able to do “1.Accessible from PC1 to PC2” in general. NAT on ER605 will not allow it.