IPhone randomly disconnects from Wi-Fi?

It's a 3gs that I've had for a couple of years and it doesn't happen that often, but it just does it randomly disconnects from Wi-Fi while I'm using it. I'll be messaging someone on Facebook and all of a sudden a notification pops up saying something about Couldn't connect to internet. I go to the Wi-Fi on the settings and there's no available networks to connect to, not my network or my neighbors'. I turn off the Wi-Fi option, I reset my phone, I reset the modem, I manually enter my network information on the 'Other' option, everything and it stays the same. My brother also has an old 3gs, but it never happens to his.
I just wanna know why this happens and how to fix it because it's freaking annoying.especially when I'm in the middle of something.

Added (1). I also reset the router*

It has to do with your internet provider - you should contact them - i've have had the same problem - I have Comcast…

Hope I was a help! Happy Holidays!

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