If I’m guessing correctly on how you hav it connected and wjat you mean by ‘switch off Humeax’
1. switch on TV
2. switch on Humax
3. use the TV source button on the TV’s remote to switch to the Humax
Step 3 can sometimes be setup to do it automatically but will depend on which TV you have and what options it has for CEC and/or auto switching through an HDMI cable becoming active.
Which TV do you have? Make? Model?
How is the Humax connected to the TV in addition to the RF cable?
Do you have other HDMI devices connected to the TV?
Is the RF cable from the Humax to the TV what you want? I.e. Do you often want to watch using the TVs own tuners?