If your router maintains logs, look at them before resetting.
Maybe your ISP is having issues with DNS servers, in which case you might try setting DNS on the router manually and use Google or some other public DNS service. Certificate errors and intermittent outages could indicate DNS lookup failures. Certificates for TLS are often validated by comparing the source IP address sending you a certificate with the address obtained from a DNS lookup, if the lookup fails, the certificate may be rejected.