Hi Shahbaz,
It would be best to get a full patch listing from the cli, simply run com installed, that should give you the exact command to list all installed patches as what you have provided is simply p9998 which is the patch going to V12.
Going to V11.5 is pretty straight forward, you simply need to install p9997, and review the release notes for that patch. You will have to review the health check log and the action you need to take.
Please review the online documentation titled upgrade path:
https://www.ibm.com/docs/en/gdp/11.5?topic=upgrade-identifying-correct-path
We would always recommend the latest V11.4 bundle and sniffer bundle, before upgrade. check IBM Fix Central for that.
Also the upgrade must be done top down method, CM, Aggregators and then Collectors.
Hope that helps.