My custom fields are not visible on Contact's Activity Log
Have you enabled detailed logging (this is disabled by default)?

If no, you can enable this from the Misc (Undelete, PDFs, Limits, Logging, Captcha, etc.) menu option -

/civicrm/admin/setting/misc?reset=1

This feature records a full history of all changes made.

Please note the below before this feature is enabled -

Detailed logging (aka "logging") allows you to maintain a full history of all changes made to everything in CiviCRM. It's a powerful feature which is turned off be default — because it's a bit overkill for most organizations, and it comes with some downsides too.

Full details can be found HERE