I'm not going to say how old I am, I'll just say I wasn't born in this century

Anyhow I keep forgetting to disconnect my Wi-Fi and leave it on and let my phone fall asleep.

Today when I woke up a message was on screen from my Internet provider telling me that I'm within 75% of my 25 gigabyte allowed bandwidth usage, but it might of said I had used 75% within 48hrs, my phone tells me I've only used 10 gigabytes.

So do I need a post it note reminder type app to remind me to shut off my Wi-Fi?

Owe I use a nickname and password to connect to my Wi-Fi modem/Router with Rogers cable.

Maybe what you need is an app that will turn it off if it hasn't been used for so many minutes. I've no idea whether there is something like that - bit like an old-fashioned screen saver from the days when we worried about burning out our screens.

I set an alarm on my phone to remind me to take my medication before I go to bed. If you're shutting it off at or around a particular time every evening, that might be a good way to go. Actual, physical, IRL post-it notes are good, too. And unless we've got a lot of 14-and-unders, I think most of us weren't born this century

I wasn't born this century either, or in either of the last 2 decades of the previous century. I need reminders for everything or I will certainly forget.

That said, on your wifi settings, Menu (3-dots), Advanced, "Keep Wi-Fi on during sleep", you can change that from Always to "Only when plugged in" or "Never (increases data usage)".

I set mine to "Only when plugged in" to reduce battery usage, but if you don't want your phone using your home internet connection when it sleeps, you can put it on Never.

In a few weeks I'll be getting a new PC and then I'll have a real PC, I'll only connect my phone to my Wi-Fi once a week to install essential updates.

I also changed my plan to 70 gigabytes earlier on today and at the end of January my cable provider has a plan of 240 gigabytes at the same price as my 70 gigabyte plan for 8 years that I'm considering.