I set my router to only allow access through HTTPS, and now I can't access it. Can I fix this without resetting the router completely?

I have Asus RT-AC87U and this happened to me too. I enabled under the Advance Settings → Administration → Authentication Method → just HTTPS. I thought that to log into my Router afterwards I just had to put in the URL bar the URL Address https://192.168.1.1

But instead it should be https://192.168.1.1:8443 or https://router.asus.com:8443.

Try clearing your browser cache or trying another browser. You should be prompted to confirm a security certificate exception when using https. Rebooting the router couldn't hurt either.