In regards to the issue you are seeing one thing I'd suggest to do is factory reset the router.
This can be done by pressing a pin, cocktail stick etc.. into the "reset hole on the back of the router for 5-15 seconds. You should see the router reboot and then within a few minutes hopefully this may have resolved the issue you're seeing.
You can also try following the steps on this thread
Let us know if you are still having problems logging in.
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