After restarting my router and the Raspberry Pi where I run Home Assistant I faced a 403: Forbidden error when accessing using to HA through the dynamic DNS url, i.e.: If I tried with the internal ip I managed to access without facing any error so I thought was something external.

After investigating it turned out to be that HA was blocking the new ip. To unblock it you just have to open the file ip_bans.yaml and remove all the items there. You should see there the new ip as it tried to access.

Once ip_bans.yaml file it’s empty restart HA going to Configuration > Settings. Then click to check configuration and if everything is fine click to restart.

To know more about it check the official documentation.