Is there away you can open a private browsing window, that has no cache, and try going to whm -> DNS Functions -> Edit DNS Zones, to see if the problems fixed?
With my system, there were two issues. AutoSSL was not generating an SSL certificate for WHM. I had to make my main account a reseller account and then owner of the domain. Then AutoSSL properly generated the SSL certificate for whm.mydomain.com. After that, it was just a matter of clearing the cache for whm.mydomain.com, then the left navigation bar was back and everything works as expected.
If you know how to view the SSL certificate for your domain, you can verify that whm.yourdomain.com has a valid SSL certificate by viewing the certificate, going to details, and looking at the alternative names. whm.yourdomain.com should be listed. If not, I suspect your server is a bit like mine, and whm is using a self-signed cert.