This is not the default address of OpenWrt, so it would seem that you're probably looking at this from an upstream network that has a connection to the OpenWrt One via its wan port. If that is correct, the inability to reach it is expected behavior -- the firewall prohibits access from the wan.
You need to connect a computer to the lan port the OpenWrt one. You should get an address on the 192.168.1.0/24 network, and the router address will be at 192.168.1.1.
Disconnect the OpenWrt One from the upstream network.
Connect a computer directly to the lan port of the OpenWrt One. (by default, that will be the non-PoE enabled port).
Make sure your computer is set to get an address via DHCP.
Open 192.168.1.1 (via ssh or a browser).
From there, you can configure the device... the changes you make are dependent on how you intend to use the router (i.e. main router, a router behind your existing router, or a bridged-AP -- each is different).
Finally, for a newbie like me, is there a guide to show which settings I should be changing and the advised settings? There are plenty of settings?
I want this device connected into my ISP router, but I want this to be the main gateway, issuing ip addresses, firewall etc. The ISP router is only in use as it has the built in ONT.
Start with the OpenWrt router and get that working first, but it does make sense to move both (~=all) away from the usual default subnets (so if you do a reset of either, there still won't be an overlap at any point in time).
There is something really wrong if your ISP router is 192.168.1.2 and the OpenWRT One 192.168.1.1, unless you have reconfigured it to AP (instead of router-) mode. Nor will the OpenWrt One magically change its LAN side address.
The subnets (192.168.x.y) on WAN and LAN mustn't overlap, ever.
…but I don't know what you're doing, as you have obviously done quite a few -unmentioned- changes over the course of this thread, so I don't know where you stand now, nor where you want to go, but what I did hear feels very wrong/ misconfigured. (There is nothing specific to the OpenWrt One here, nor even to OpenWrt, this is just basic network design/ constraints).
slh, you guys are the experts, I am the amateur. I just want to use the ISP Modem/Router as a device to convert the fibre optic to an Ethernet port that I can connect my One router to. I don't want it to do anything else (like DHCP). What ip address do you suggest I assign it?