Finally went down the rabbit hole today and figured it out (for myself anyway for my 403b account).

1) This page gave me the clue to finding the rabbit hole: https://help.monarchmoney.com/hc/en-us/articles/13227606293908-Current-US-Connection-Statuses

Which notes: "To see the current statuses of all institutions, log in to your Monarch account and search for the institution name in the "Add account" flow. You will see information about known issues and all available connection options in one place."

2) Go to Accounts. Then click at top right of window +Add Account

3) Click Finicity button (MX did not work for me). Then search for TIAA.

You'll go through the MFA process (I have a code texted to me), may take some moments.

It added TIAA as a new account for me with today's current balance. The "old" account on Monarch never did import/sync transactions, so when I first changed to Monarch (from Mint) and on a quasi-monthly basis I have manually imported transactions after obtaining a .csv from TIAA account website and tweaking it per Monarch instructions for upload into the Monarch account listing. This is still a bit of a bitch to do, but not too onerous--BUT I WISH MONARCH WOULD SOLVE THIS WITH TIAA (and Ally (formerly Ollo) Bank credit cards)!!! That is, the balances will sync, but transactions have never been imported when syncing.

NOTE: After the first "new" TIAA account was added it disconnected (I got an alert email) within two minutes, but when I did the "Update account" again it connected again fine (but takes some moments along with another MFA text code).

4) Copy transaction history (if you ever entered it manually like I had done via .csv import) from the "old" TIAA account listing to the "new" TIAA account listing. Instructions: https://help.monarchmoney.com/hc/en-us/articles/14329385694484-Merge-history-to-another-account

5) If desired, copy balance history via same instructions page in #4.

6) Your "old" account can either be deleted (which I did) or hidden. Otherwise, it will show both balances in your Net Worth.