Read the release notes for R12.4.0 here: https://supportforums.adtran.com/docs/DOC-8458
See top of page 8. There is a problem with generating TLS certificates. Workaround is to boot it on R11.10.7 first, then upgrade.
Also the OpenSSL version was upgraded to 1.0.2k which may have disabled older insecure ciphers. Try a different browser or upgrade yours to the latest.
What if any error pops up when you try to connect?
I had it on ver 12.3 everything worked, then we I upgraded to 12.4 I could no longer connect. Hope that clarifies. Have tried different browsers.
Load R11.10.7 and reboot. Verify that HTTPS works. Then upgrade to R12.4.0. See the release notes linked earlier.