Hello,
I have tried to replicate your scenario: enabled remote HTTPS access, configured port forwarding to a LAN device and DMZ for that same device, but the router remained accessible, which makes it difficult to tell, what could have caused it, and, if you could even access it locally.
Are you able to send SMS to your device? Some configuration changes could be attempted via SMS.
Do you have the router added to RMS? Maybe it would be accessible via the platform. You could try to add it to the system, if it has not been added before, as each new device gets a month's worth of service. This might allow for a remote recovery.
Best regards,