First Republic Bank occasionally has issues connecting to third parties due to multi-factor authentication. First Republic Bank is urgently working on a fix for this, which is slated for Q4 of 2021. In the meantime, there are two options to maintain your credit limit in the meantime:
Upload monthly PDF statements via your Brex dashboard (screenshots are not allowed).
Obtain a set of read-only credentials that do not have this MFA/token on them. First Republic Bank can help you obtain these read-only credentials.
Note: The success rate is not 100%, but if your FRB account is currently using a token, then this workaround has a high chance of working. Please ask your FRB rep about this option.