Same issue. You'll never fix it and waste half your life trying, but I found two workarounds that should put an end to your stress. I tried everything possible to get my optimum account loaded onto my Outlook with no success. The annoying thing, was that it worked fine for years, then, "poof", just stopped working one day and has been impossible to re-connect ever since. MS support offers nothing but asking additional questions then having you run through time consuming "tests" that never yield results. Optimum support is virtually non-existent.
Workaround 1: I stumbled over Mailbird, an email client that connected my optimum account flawlessly. It has its limitations also, I think allowing 3 mail accounts and there may be others (I just started using it) but it's such a relief not to go into my browser every time I need to fetch email, I just keep the Mailbird pinned to my taskbar next to the outlook.
Workaround 2: Create a new (NON OPTIMUM) email address that will load onto your Outlook, then open your optimum account through your browser, locate preferences, and select "Mail Forwarding". Enter your newly created email address in the specified field and you will now get your Optimum mail through your Outlook App.
Neither are perfect but I hope this provides you the relief I finally found after months of trying to straighten this out with no success.