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Make sure you qualify for this method. Determine who your ISP is. Look up your ISP's tech support number. Call your ISP. Navigate through the automated prompts. Present the representative with your account information. Ask the representative to reboot your modem.
If you're renting your modem from an ISP (e.g., Xfinity or CenturyLink), you can try calling your ISP and asking them to reboot your modem. Not all ISPs will do this for you, but it's the only way to reboot an ISP-owned router when you're away from the network. This usually won't work if you aren't renting your modem from an ISP. If you don't know who your ISP is and you're away from the modem, you can probably find out by looking at your online cable bill or charges on your credit or debit card statement. You can do this by going to https://www.google.com/ in a web browser, searching for your ISP's name and the phrase "customer service phone number", and reviewing phone number(s) in the search results. For example, to see Comcast's phone number, you would type comcast customer service phone number into Google and press ↵ Enter to see the phone number at the top of the search results. Dial the customer service number to do so. This will usually result in an automated greeting. You'll have to indicate that you want to speak with a representative before proceeding; this process will vary depending on the ISP, so follow the spoken instructions until you reach a real person. When asked, tell the representative your phone number, your account number, and/or your address. Not all ISP representatives will require all of this information. Similarly, some ISPs will need more information (e.g., your date of birth) to confirm your account. If they can reboot the modem, they should do so, though they may ask you to confirm some account information before they officially reboot your modem.