You can chat with Mint support at the link below.
How do I contact support? (intuit.com)
Otherwise, there is really no support for Mint on this board.
I am having the exact problem. It's been going on for over 6 months.
After chatting with Mint support, they suggested making the old account inactive and readding with "nb.fidelity" in the account search field, which is working for me.
I did the exact same steps - marked my existing Fidelity accounts as inactive and added using the new nb.fidelity website and my IRA accounts are missing, I tried to re-add but no success. Not sure on which side is the problem. BTW - I am still able to see all my Fidelity accounts from my Chase app so likely the issue is with the Mint app.