This solution finally worked for me to be able to access Dockers with custom IP addresses (much easier than VLAN setups, adding NICs, and is not limited to just one docker (i.e Pi-Hole) like the solution that replaces the default port 80 with Pi-Hole for pure DNS.
However, it was still a little hard to understand and follow how to configure this in my network -- and with an Edgerouter (ER-X). So I thought I'd go ahead and share the specifics in case anyone else could use some additional clarity and/or has an Edgerouter. After mucking around with my firewall rules and getting back out of the wrong rabbit holes, it really was fairly easy in the end.
Re-Cap of setting up WireGuard (in Unraid) for access to Docker containers with Custom IP Addresses (e.g. Pi-hole, Unifi, etc.):
First, in order to access Docker's that have a custom IP Address you need to disable NAT in the Wireguard VPN settings... as noted in the just-gotta-know-where-to-look on-screen documentation:
So, once you disable NAT in the WireGuard Configuration (Settings -> VPN Manager), then Unraid provides the key details in a remark for you:
As noted also in the documentation comments and the remark you will need to configure a staic route on the router, but there is still an important missing detail from the on-screen info. Luckily these have been outline in the "WireGuard quickstart" forum thread (linked here). These details (along with the post quoted above) really helped with some additional details that made it more clear for me finally get my network properly configured -- and as it turns out is really very simple. The "WireGuard quickstart" guide is actually a different forum thread, but I found this thread before I finally found my way over to that one (via more Google-Fu) . . . so it seems useful to provide these details here:
The quickstart guide (thread linked above) was updated as of February 20, 2020 with a section labeled "Complex Networks" and it provides the key details that were most helpful.
How to Configure Static Route for WireGuard (on Unraid) in an EdgeRouter (ER-X):
The remaining element was to sort out how to correctly set up the static route in an Edgerouter (ER-X).... so for those that may find their way here and have an Edgerouter (or similar), here's the process:
1. On the Routes Tab, click to Add a new Static Route:
2. Enter the details that are provided by the Unraid UI (see screenshot above when NAT is disabled under VPN Management):
3. Save and Apply the changes to the EdgeRouter... the end result will look like this if you open it after saving:
4. And Finally, you need to ensure that Docker is correctly set to enable "Host Access to custom networks" in the Docker Settings (Settings -> Docker):
Which will result in this... Actual url to detailed instruction is just below:
Note: The original details for this step are on Page 8 of this thread, but I found them from the link that is also on the "WireGuard quickstart" thread.... so I'm posting here again for continuity: