for an existing monitor you can click on "edit" and select "Frequency and locations" in the monitor settings menu.
There you can change the frequency. For http monitors it is possible to choose between 1 min and 60 min. For browser monitors it is to choose possible between 5 min and 4 hours.
Please check the environment API v2. There you should have an option to trigger on demand executions:
I already linked to the documentation in the other post. You might be able to solve it via a script, so that you trigger the monitor via the API every 8 or 12 hours.
As a work around you can also set up a reoccurring maintenance window that will suppress runs at a certain time. then allow it to run at another time. Dynatrace does not have the ability to set larger intervals as you are requesting, you can put in a RFE for that function though and maybe it will be provided in the next few updates