Why does my internet only work in certain places?

According to GSMArena, your device (LG K7) uses Wi-Fi 802.11 b/g/n standards. According to my findings, this standard tends to have limited coverage on some bands.

More explanation on different wifi standards has been described here:

My suspicion is that probably something/someone messed up the router settings to operate on a certain band, perhaps the 5GHz one or another. This normally covers a pretty small area, but is associated with such high speeds and is less congested. On the other hand;

The shorter waves used by the 5 GHz band makes it less able to
penetrate walls and solid objects

I suspect this may be the reason you can't get exclusive connection (instead you get disconnections and reconnections) the moment you move further away from the router.

Consider this illustration:

It gives a clear picture on extent of Wi-Fi coverage on common bands.

It may be the way round that you unknowingly tweaked these specific settings.

As a workaround You may need to revert to default configuration either on your router or your device and see if the situation changes.