How to prevent internet connection from disconnecting itself?

Have you tried changing out the cables? I'm assuming a broadband setup here with a modem and a router. Even if you're connected straight into the modem from the computer, the Ethernet cable could be bad. That's a cheap troubleshooting step: buy or find another cable and swap them out. See if that solves the problem.

If not, the next thing to swap would be the modem, since this shouldn't cost you anything. Ask your ISP for a new device. If that fixes the problem, solved and you're not out anything.

The next thing to do, before you buy a new router or any other hardware, is to have someone come over with a laptop and see if the same thing happens to them. Have them check connected with an Ethernet cable and using wi-fi, if it's part of the equation. If they have no disconnect issues, then the problem is in your hardware.

if this is a desktop PC, try troubleshooting the network card first. If you have an integrated Ethernet card (built into the motherboard), you could buy or borrow a separate Ethernet card and install it in one of the empty slots. if tour connections are good, then your Ethernet circuit on the motherboard is defective. You could continue to use the new controller or replace the motherboard.

If you already have a separate Ethernet card, try installing it in another slot. if that doesn't help, try a new one...they're pretty cheap.

The trick here si to eliminate the stuff that will cost your little or nothing. Start with the cables and then work your way through the rest.