Thanks trendy. I realised that once I looked at the WG Client setup wiki.
I didn't have any luck getting that far though. Here's what I did:
Step 1: Booted into a Live version of Ubuntu (I always use this for testing to eliminate configuration problems, good friend taught me this trick)
Step 2: Connected my PC to LAN port 1 of the OpenWrt router
Step 3: Logged in to the OpenWrt router using SSH
Step 4: Logged in to the OpenWrt router using the Web portal
It all works well up to here.
Step 6: Connected OpenWrt router WAN port to ISP router LAN port 1
I gave the OpenWrt router some time to make a connection then I started following the WG Client guide starting with the first step, updating the packages.
This doesn't work.
wget returned 4.
Check your network settings and connectivity.
I left the cables where they were and tested the Internet connection using Firefox. I got the standard error message when there's no Internet connectivity. I waited some more to make sure the OpenWrt router had enough time to make a connection to the Internet and tried again but it clearly wasn't going to work.
Here's what I did next:
Step 1: I disconnected the OpenWrt router from the PC
Step 2: I disconnected the OpenWrt router from the ISP router
Step 3: I connected the PC directly to the ISP router LAN port 1 - I get a connection this way, and that's using the same cable as between the OpenWrt router and the ISP router and the same ports on the PC and the ISP router.
I made some small changes to Settings on the ISP router when I got it last year but I think they're harmless. I turned off UPnP in the Firewall because I read this was unsafe.
I made some changes on the OpenWrt router after I flashed 19.07.3. I disabled IPv6 in the Firewall by unchecking Allow-DHCPv6, Allow-MLD, Allow-ICMPv6-Input, Allow-ICMPv6-Forward, Allow-IPSec-ESP and Allow-ISAKMP. My ISP doesn't do IPv6 and it's disabled on their router anyway. I also stopped the WAN6 interface from coming up on boot, deleted the IPv6 ULA prefix, disabled bultin IPv6 management on the LAN and WAN interfaces, disabled some settings in the LAN DHCP IPv6 Settings section, and added net.ipv6.conf.default.forwarding=0 and net.ipv6.conf.all.forwarding=0 to /etc/sysctl.conf.
So here's where I am:
ethernet cables are working, SSH and Web portal connections to OpenWrt router from PC are working, direct connection to the Internet from the PC through the ISP router is working.
OpenWrt router connection to the Internet isn't working, PC connection to the Internet through the OpenWrt router connected to the ISP router isn't working.
That's all I can do just now. Should be back in a couple days to see if there's any replies.