Hi so it turns out no one on the internet has had this problem which means it should be something really easy to solve... or quite the opposite.
So essentially I have set up a minecraft server for me and my friends to play on and everything works perfectly fine. People are able to connect, except it requires one small caveat, unless they are the first to hop on the server, in order to connect, they require me to hop on the server myself and disconnect in order to seemingly allow them to connect themselves. It's doing my head in trying to solve it. I don't remember every having this issue five years ago when I first made one, and so I don't know what's happening now. I even helped another friend set up their own one and I was able to connect to theirs without issue.
My instinct told me it was a firewall issue, however I have played around with everything I can think reasonably would affect it, but have yet to do anything to actually fix it. I also don't want to keep playing with things I don't understand unless I mess with something I shouldn't be messing with. I honestly don't know what the issue is or how to solve it.
So if there's any way you can help it would be greatly appreciated, and if there's any further info you need I will be happy to provide.