I don't think the issue is a missing modifier. Add on codes don't require them. The 99354 is approved to use with the 90837 but they do not allow the first additional 1 - 29 minutes in addition to the 90837. Since it was a 111 minute session, the 99354 should be allowed, but TRICARE may not allow for additional time. I would recommend calling TRICARE to find out if it is a TRICARE exclusion, or if they require something indicating the amount of time spent.
Also, if this was a crisis you may want to consider using crisis codes 90839 and 90840.