Why isn't my Wi-Fi working on my devices, after not using it for a few months?

My Wi-Fi used to work fine, but, after being away from home for 6 months, I have come back to find that it is extremely slow on all of my devices (to the point where watching a video or downloading anything is out of the question), however it works fine on the rest of my family's devices (who have been here while I've been away).
My devices (iPhone 6s, MacBook Pro and xBox) work perfectly fine on any other Wi-Fi, and the phone and computer are either the same or newer than my family's, so I can't understand why I'm having problems but they are not.
My xBox, which hasn't been used in the time I've been away, just straight up will not connect, and the phone and laptop are cripplingly slow. It's a big problem as I need them for work.
Does anybody know why this is happening and how I can fix it?

Either something's up with your plan, or you may need to restart your router. Best thing I can tell you is contact your provider since you most likely don't know. (if you do know restart the router)

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