It isn't always easy to nail down the exact cause of a modem resetting, but if you follow the right order of troubleshooting, you should be able to resolve the issue.
Cause of Your Modem Resetting
Your incoming internet connection from your ISP goes directly from the cable in your wall to the modem. Your entire home's internet connection depends on that modem. When it unexpectedly restarts, it doesn't matter how great your mesh system is, how great your wiring is, or even the "perfect" placement of your W-Fi router—all your work and your productivity at risk. Your streaming TV and movies will stop, too.
Troubleshooting your modem to stop random restarts should start with hardware first—things like cables and power. Then work your way through software and setting issues. You will hopefully be able to fix the problem without having to replace the modem itself.
How to Fix it When Your Modem Keeps Resetting
These fixes apply to all models of modem regardless of your Internet Service Provider (ISP). Some menu options may vary, so check the modem manufacturer website for details.
Check the power connectors. A loose power connection is the most common reason a modem may keep restarting. The loose connection could be at the outlet or the power port on the back of your modem. Watch the power lights on the front of the modem as you wriggle both ends of the power connection. If the light flickers, changing your modem's power adapter could solve the problem. Additionally, it's always a good idea to plug a modem into a surge protector or uninterruptible power supply to maintain steady, reliable power to your modem.
Check the cable connections. Maybe the coax cable connection on the back of the modem is loose. Coax is the large, round cable that goes from the wall or outlet into the modem's port. It's the primary source of internet for your entire house. If you have internet through your phone provider, this will be a DSL cable instead, which looks a lot like a regular phone line. Ensure you've securely fastened the cord to the input port on the back of the modem, and there aren't any breaks or cracks. If there are, look into replacing it (you may need to contact your ISP for help with this).
Make sure your modem isn't overheating. If all connections look good, you'll need to start looking at the modem itself. A modem will get warm during everyday use but shouldn't feel hot to the touch. If it is, then it isn't correctly cooling during use. A few things that can help are making sure there's plenty of space (at least 6 inches) around the modem for air to pass. Secondly, clean off any vents in the modem case so air can easily flow through.
Your modem may be overworking, which often happens when the modem also has an included router feature. In this case, the router functions handle apportioning IP addresses to all of your network devices. If you've added smart home devices, mobile devices, and computers to your network, the modem may be at its functional limit, which can either slow it down or cause it to restart entirely. To fix this, turn off your modem. Then turn off all devices using the internet, including smart home devices, all mobile devices, and all computers. Then restart the modem. Try turning on one device at a time to test if the modem stays up. If you do find that it restarts once you reach a certain number of devices, you may need to talk to your ISP about upgrading to a new modem or buying a higher-end model yourself. You can also try the following troubleshooting tip to see if it resolves the issue.
If you use a combination modem and router unit, changing the Wi-Fi channel you use with your devices to connect to the modem/router can help. Having too many devices connecting on one channel can congest the wireless network, slowing down the modem or cause it to restart. You'll notice a 2.4 GHz band and a 5 GHz band available by default with most modem/routers. Choose the best wireless channel for each device, depending on its capabilities (not all devices can connect to 5 GHz). Try to divide the devices evenly across the two channels to see if this resolves the modem restarting issue.
Reset your modem. If you've come this far and your modem is still restarting intermittently, it could be a setting or some other feature changed from default since you first installed the modem. Performing a full reset will set all of these back to normal. Keep in mind a reset is more drastic than a reboot because it will remove all passwords, DNS settings, ports, and firewall settings that you might have configured.
Update your modem's firmware. Modem manufacturers sometimes update the software, which makes the modem work properly. These updates include security patches, software bugs, performance updates, and more. If your modem doesn't receive those patches, fixes (like resetting), you won't get to take advantage of the software's fixes.
Re-sync your modem. If your modem is still restarting after you've reset it, one last thing you can try is making sure it's synced with the network. A desynchronized modem can occur if your modem has been left on and running for an extended time. Desynchronization can lead to more corrupt packets or lost data packets. You can fix this by contacting your ISP and asking them to check your connection; they may be able to reset it and restore your internet connection quality.
Buy a new modem. If none of the tips above resolved the issue with your modem restarting, you might need to replace the modem. Again, your ISP may replace your modem for a fee, or you can upgrade to a higher quality modem by buying it yourself. Just check with your ISP to be sure you're purchasing a compatible model that works with their service.