I have had a similar issue with a customer so may or may not be related.
In version 7 or 8 firmware, factory defaulting a unit deletes the cert01.pem which is the certificate used for HTTPS.
With these firmware versions the cert 01.pem file is dynamically created after factory defaulting if the time on the unit is after 00:00:00 1st Jan 2019.
However, if the unit has never had its time set, its time will be however long the router has been powered since 00:00:00 1st Jan 2000.
You can set the time with "time 00 00 00 01 01 19" via SSH or Telnet.
I suspect you may have another issue though if the above doesn't work.