Spectrum/Charter MacOS Mail Doesn't Work

Recently I had to change the password for my ISP (Spectrum, formerly Charter). Of course all my IMAP email clients stopped working. Changing to the new password in IOS for my iPhone and iPad went effortlessly. However the MacOS Mail app on both my iMac (Sierra) and MacBook Pro (El Capitan) would not accept the new password. I tried everything I could think of including deleting and recreating the email accounts. I also deleted all prior Charter entries in Keychain Access. The previous Charter setup used imap.charter.net and smtp.charter.net as the servers. Spectrum's new instructions are to use mobile.charter.net for all clients, including desktops. This works on the IOS devices but is rejected on my Macs. If I try to use the old servers, I get a warning that it can only make an insecure connection. If I accept that it appears to connect successfully but I'm nervous about not having a secure connection. Any ideas? Has anyone gotten MacOS Mail to work with the mobile.charter.net server?