Yes a "modem" interface is unnecessary here, as the router already holds an IP in 192.168.0.0/24.
Since the wan is the default route, any IP outside 192.168.1.0 (and other networks the router knows about) will go to the wan interface, so access to 192.168.0.1 and other 192.168.0.X is inherent.
The preferred scenario is to use the ISP modem only as a source of Internet, and move all LAN devices to the other side of the site's main router (OpenWrt). If you can't or won't do that, symmetric routing can be set up so the 192.168.0.0 network has access in both directions to the 192.168.1.0 network.
I tried many things and made changes, but the issue remained.
All modifications I've made to the WAN interface have caused me to lose my internet connection, and LAN or MODEM interface adjustments have caused me to lose contact with the Openwrt router.
What would the settings be for the WAN device (where the internet comes from the isp modem through cable), followed by the settings for LAN and MODEM, if I wanted to start over?
Yes, because defining two paths to the same upstream network 192.168.0.1/24 is just plain wrong, and putting one in wan and the other in lan id IMHO doubly wrong.
What is your actual goal here:
If you just want to reach the docsis modem under 192.168.0.1 just remove your MODEM interface and things should magically work
If you actually want bi-directional traffic flowing between machines connected the ISP router (be it wired or wireless) you should configure your OpenWrt device as "dumb AP" (search for the term, there are recipes for that on the wiki).
So, let's start at the beginning.
I require access to four devices that are linked via cable to the Virgin Media UK isp modem at 192.168.0.1.
Via my Openwrt router, which has the IP address 192.168.1.1, I need to access the devices with the 192.168.0.x address. That is my goal.
I cannot enable the "modem mode" on the isp modem since I need to maintain the wifi active for location coverage.
OK, I take it attaching these to the OpenWrt router is not a real option?
Is this unidirectional, so do you need to reach those from devices in the 192.168.1.1/24 network or do these 192.168.0.1/24 devices also need to be able to rech devices behind the OpenWrt router?
That should already be possibly if all you do is to reset the OpenWrt router to its defaults and then configure the OpenWrt wan to use DHCP, that should be all that is needed (unless the devices in 192.168.0.1/24 also need to be able to initiate connections to devices in the OpenWrt network.
Well, "need" is a strong word here. You are IMHO making decisions and you need to weigh the trade-offs. Now, I am not about to tell wat to do, but keep in mind that the more restrictions you bring the harder it will be to find a solution.
But maybe you do not actually need an OpenWrt router but just an AP to increase the virgin routers WiFi coverage?
I can now reach every device on 192.168.0.x.
Now, how I would reach the openwrt device (192.168.0.73) from the main network? I attempted to connect to 192.168.073 from one of my computers at 192.168.0.2, but it failed.
You would need to disable the firewall (but keep the NAT) intact. Or if you only need access to a few dedicated machines use port forwarding rules... but really have a look at configuring your OpenWrt router as dumb AP instead...
I agree, but that alone will not fix anything... to access the machines in 192.168.0.1/24 the modem interface is not needed at all (as the OP has already shown) and for accessing the OpenWrt-LAN side from machines in 192.168.0.1/24 it is not enough... really the options are:
port forward and access rules for all OpenWrt LAN host that need to be accessed from 192.168.0.1/24
disabling of firewalling and NAT in OpenWrt (sort of dumb AP)
moving the machines in 192.168.0.1/24 into OpenWrt's LAN
You're correct of course @moeller0 . I'm guilty of not looking at the full thread properly and noting that he had both WAN and modem interfaces on the same subnet and that the modem is not set to bridge mode.
Having two separate interfaces on the same subnet is of course never going to work.
The only rationale I see for not wanting to put it into bridge mode is the statement that he needs wifi on the Virgin Media Hub 4.0 "active for location coverage".
So, @azarok - are you meaning to imply that one router has insufficient wifi coverage and that without both the Hub 4.0 and the Openwrt's wifi, you don't have full coverage / have dead spots?
If so, then @moeller0's suggestion of just using the Openwrt as a dumb AP to complement the Hub 4.0s wifi coverage is the right solution.
If not, and one router supplies sufficient wifi coverage, then you absolutely need to put the Hub 4.0 into bridge (modem) mode and use the Openwrt's radios to supply wifi coverage
So, @azarok - are you meaning to imply that one router has insufficient wifi coverage and that without both the Hub 4.0 and the Openwrt's wifi, you don't have full coverage / have dead spots?
The router and Hub 4.0 are mounted at the very end of either side of the house.
None of them can completely cover the house on their own.
