I suggest you either offer the router on your local Freecycle, Freegle or other similar group. If you have no such group then send the router back to Plusnet. As they are owned by BT/EE then you could probably just hand it in at your local high street branch. If none of these option appeal to you then your local recycling centre (tip) is probably its final destination.
You can connect devices to it via wifi or ethernet and use it to control a local area network. This could include printers, media streamers, etc. However, none of the connected devices will have internet access unless you have a current broadband account with Plusnet. So media streamers which stream locally stored files will work but services from Netflix, Apple, Amazon, etc will not be available.
Edit: It is not possible to tether this router to a phone and access the internet over 4/5G. Other routers may have this capability but ISP supplied routers rarely do. They are built for minimal cost and whatever functions they may have beyond what the ISP deems a minimal requirement is usually locked down to prevent customers accessing the settings.
It might work with a different provider, maybe? But for sure you are going to need a SIM to get data. You would be better off in one of the router groups. You could also search the internet with the terms dd-wrt or openwrt after the router name.