trying to reclaim some disk space by truncating SQL Log files

Hi Spiceheads - hoping you can help.

I'm running SQL Server 2016 and having a problem truncating logs to free up disk space.

here is what I've done so far:

1. right click database select Tasks and then Backup

2. Changed backup Type to Transaction Log and pointed backup to another local drive

3 Under Media Options Transaction log - I chose the Truncate the transaction log.

4. ran the backup job thinking that this would do the trick but the transaction logs appear the same size and no disk space is recovered.

So I ran a this query to see what is holding on to the logs:

SELECT log_reuse_wait_desc FROM sys.databases

and it returns - OLDEST_PAGE

So what do I do now?

The database is running in full recovery mode - do I switch to simple recovery and then switch back. What does this corrupt the recovery points for the database?

Thanks in advance