Welcome to the Kitz forum.
We all have started at that experience level in our past history . . . so don't be concerned.
It would be quite helpful if you will please explain, in your own words, exactly what equipment you have and how you intend to connect it together.
Thanks for the welcome! In short, I want to have a Ubiquiti unifi set up comprising this modem, a USG, a POE switch and two Unifi access points.
That's good. Once it has been unlocked, there is nothing else that a novice should do with it. It is really "plug-in and use" device in its default configuration. If you have made changes to the configuration, via the GUI, they can be reverted by pressing the reset button and holding it for, say, 10 to 15 seconds. (That is what I call a "long reset".)
I have tried many variations of the reset but none enable me to get back to the web GUI (but I can get back to the bootloader page). I must have re-flashed the firmware ten times now, but I can't get back to the GUI page. I've disabled antivirus and windows defender.
I didn't (as far as I can recall) make any changes via the GUI a month ago, when I could access it.
Also, because I didn't make any changes, I also didn't put any username or password in to it (e.g. homehub@btbroadband.com).
The DSL port should be connected to the telephone line, either via a centralised filter or a plug-in "rat's tail" micro-filter. The LAN1 port is where the other networking equipment would be connected. (That explains why I asked for a description of your equipment, above, as I have no idea of your plans.) It is via the LAN2 port that one may access either the GUI or the CLI, using the 192.168.1.1 IPv4 address.
I didn't connect a microfilter. I simply disconnected the DSL line from my BT Homehub (SmartHub, I think) and plugged it in to the HG612. The line seems to simply go straight in to my Openreach box without a filter (does it have a filter built in?).
When I did this, the DLS light did
not come on. Could this be because I didn't put any username or password settings in to the HG612 a month ago?
Thanks again for your help.