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Welcome!What Hardware version is your router? Look at sticker under router.
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Firmware version is currently loaded? Found on routers web page under status.
What region are you located?
Are you wired or wireless connected to the router?
Has a Factory Reset been performed?
Check ISP MTU requirements, Cable is usually 1500, DSL is around 1492 down to 1472. Call the ISP and ask. Link>
Checking MTU ValuesFor DSL/PPPoE connections on the router, ensure that "Always ON" option is enabled.
Ensure the phone lines for the DSL service are in good working condition, signal is at proper levels from the ISP and ensure the DSL phone line is filtered correctly
Ensure the other Cisco router is not interfering with NAT issues as having 2 routers on the same system can cause problems.