You need to systematically work from the modem outward in your network.

In the best case you get a failure that stays broken or happens enough that you can tolerate waiting for it to happen again.

To prove it is not your equipment plug a PC directly into the modem when you have a failure. If the PC does not work then you know its the modem or the vendor.

If this works it is likely a issue with your equipment and it gets tougher. On your router during a failure you want to test both wired and wireless ports. You also want to see if the router itself can see the internet. Most routers can ping stuff so try something like or another address you know will respond when it is working.
If this does not work and/or of the wired ports do not work you have a issue with the router. Firmware upgrade is a last resort before replacing it.

If you only have issues on the wireless you have the main reason I do not recommend people use wireless unless they have no other option. Wireless issues are almost impossible to find, they operate on unlicensed bands so you never really know what will interfere. When it works wireless works well when it doesn't only a professional with expensive equipment can tell you exactly why it does not work.

If your problem is the more common one that your network is not down it is just slow certain times of the day that almost always is traffic in the ISP network but if you push them they will point out that 25m is a peak number and that they could deliver .001m and still not be in violation. They make their money by overselling the network and hoping everyone does not try to use it at the same time.