Mix return means you are sending vocals to the FX1 mix, and getting a signal back on FX return1.
FX return 1 fader is then controlling the amount of reverb in each mix.
So – get the FX1 return fader (either on the top layer or the custom layer) and select the LR mix.
Then put the fader up and down, check that the FX are being modulated in the FOH mix.
Now select Mix1, the fader should jump to a new position – this is the send level to Mix1 – bring the fader down and the FX will disappear from the monitor. Repeat for all mixes.
Select LR and dial in the desired level of FX, this will not affect the sends to monitors (unless they are both post fade, and have some level)
If you want no fx in the monitors at all then you can disable the FX return routing to those mixes, select the fx return fader, then the processing screen …