Unfortunately, TREP APIs does not support requesting data by ISIN. Only RIC can be used to request data.
If you don't have Eikon access - or you are working on a non-desktop based implementation, you may also be interested in our Refinitiv Data Platform and its associated APIs and Library.
RDP would allow you to do symbology conversions as well as consume realtime + historical data etc.
Please reach out to your Refinitiv Account representative to discuss RDP if interested.