Can't set up Charter email account in Apple Mail

I was very hopeful when I read Tesserax's advice back in 8/20/17 to a similar difficulty of jfrillici but even with a new user account I was unable to get past the first window requiring the account address and password (both of which worked in logging into Charter.net) even after option-clicking the sign in. This blockage is only occurring on an iMac and not with the same account info on both an iPad Pro and an iPhone. Is there anything else I can try? I've also spent unsuccessful time on the phone with a senior Apple person who had control of the imac and was unable to create the Charter email account.