Important information, thanks for sharing.
I want the One router be the main device so it will do DHCP, I have turned off DHCP on the ISP router.
I didn't see how to restrict the range of numbers, previously I reserved the first 30 or so numbers for devices I have assigned static IP addresses.
So for ISP router should I assign it 192.168.0.1 and for One router leave as 192.168.1.1?
Thanks
means you have to set wan IP on the One by hand, leave it on.
if the ISP router can't be bridged, don't touch it.
you can assign whatever you want, as long it isn't in the 192.168.1 subnet.
192.168.0.1 is fine.
To be clear, you made the OpenWrt WAN IP 192.168.1.2 (and not LAN), correct?
To be clear, the ISP device is still in routing mode (just with DHCP disabled), correct?
One router is 192.168.1.1 ISP router is 192.168.0.1
I tried to put ISP router into bridge mode but it didn't work. Then i couldn't access it as no internet, so I had to reset it. I couldn't access the One router even when computer direct connected to it (ethernet), so i am pretty much back to square one. Thinking about adjusting the router with a hammer just now
Or just daisy chain your devices in router mode:
ISP_device <> OpenWrt_WAN_port
The OpenWrt will get an IP/Internet from the ISP device. This should have worked without making any alterations.
Thanks. I have just spent the last few hours setting everything up with the help of ChatGPT. All seems to be working very well now. Good to chat to a machine, as I am a real newbie at this and need to be spoon fed. Asking lots of questions, would have driven the forum mad!
Just need to set up a guest network and a IOT network, test it, and if all good get 2 more units.
In that case, it's probably worth resetting to defaults and asking us for help. Or, at the minimum, post your config for review.
AI is almost never correct with the configuration of OpenWrt, and chances are that you've actually introduced config issues as a function of using ChatGPT (unless your use case is really simple).
DO NOT attempt to do this with AI. Use the documents provided by OpenWrt.
(if you intend to use ethernet to connect more devices (including APs) to this new guest network, a few quick changes to the resulting config will allow that to work).
For me it was difficult to figure out how to see the config files and copy things from it. In my CLI of the OS (zutty) I wasnt able to copy multipage-outputs to put it in here. So I moved over to vscodium where I have a ssh cli also. With that, I was able to copy the output to paste it here.
Remember to redact passwords, VPN keys, MAC addresses and any public IP addresses the different outputs might have:
cat /etc/config/network
cat /etc/config/wireless
cat /etc/config/dhcp
cat /etc/config/firewall
A good source is this page here: https://deepwiki.com/openwrt/uci/3.1-command-line-interface under section "examples" you will find hints.
For me personally its quite hard to get an overview. The most important thing might be at the moment (for me) to know, where to find the info from behind the scenes, like config entries, or maintenance calls like
nft list chain inet fw4 forward_lan
where some can see things en detail. Luci is not really helping with troubleshooting. 
And always save backups after the milestone-hings are really running! 
Good luck from a noob!
Yes, after making changes that work, it gets backed up! Just like mega spreadsheets, always backup every so often, just in case.
Happy with my new router (though coverage not as good as previous devices).
I have made a donation to openwrt and ordered 2 more units.