Just checked the currently connected devices and they are all on the 5GHz band, so I disabled that to see if it would work - it didn't, and I had to factory reset just to get the internet to show up again. I even altered SSIDs for both bands - that didn't work either.
So I think my 2.5GHz is just not working for some reason, so I am assuming my mum's phone and my brother's Wi-Fi adaptor on his computer are only able to connect to 2.5GHz bands. I don't know how to fix this issue...