When you connect to the internet, the router collects information from your provider(ISP) for your settings, this is actually a 2 way street, the provider collects information from the computer or router thats connected to the modem(routers are basically small computers anyway). It sounds like your ISP is stuck on the last settings that worked.
A hard reboot of your modem will most likely take care of this, unplug the power from your modem and leave it disconnected for a minute or two(rule of thumb is actually about 30 seconds), plug it back in and hope for the best
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I had something similar happen a long time ago(years) and had to leave mine unplugged during the day, while i was at work so that I could get online again. I think it used to be a much more common problem, Im still using the same provider but now, i can replace my router with a different one any time and everything is fine.