The problem with the certificate used by a Peplink router is not a problem, just a mis-understanding. Digital certificates enable two things: identity verification and encryption of data in-flight. Regardless of this error, you will get the in-flight encrypted data. The error has to do with the ID verification of the router. No certificate is ever issued to an IP address, so any access to a secure web page by IP address will generate this same error.
If you use HTTP, no error. Better though to stick with HTTPS.
BAD_CERT_DOMAIN means bad certificate domain and what is bad is that the certificate was issued to www.captive-portal.peplink.com rather than to 192.168.50.1 (or whatever you might have changed the IP address to). Blame your confusion on Mozilla for their poor explanation of the situation. Not that the Chrome browser explains it well either.