MySQL 8.0 and later supports virtual columns and indexes on virtual columns. See if your DBA can add a virtual column that converts the timestamp to a datetime, and put an index on it. They will probably have to write a function to do the conversion as MySQL will not allow datetime’s to be directly handled in the generated expression due to interactions with the user timezone setting.
Another way might be to use a proxy database, like DuckDB or Druid, to query the MySQL database and add the converted column.