Hey @Rness .
There is not a way to turn off or separate the frequency bands and that is by design so your devices connect to the best option for their setup and configuration. The below article goes into detail about the bands:
Also, the connection the device makes should not affect the speeds of the devices unless a setup issue is present. Most slowness issues are a result of there not being enough bandwidth but other factors can be present. If you are wanting to separate them because of slowness issues, please refer to the next article. It should assist with narrowing down the cause: