@Willy
Select the particular monitor mix you want to by pressing a Mix key (blue keys down the Right far side). Channel faders are now representing your monitor mix.
Assign each channel to either pre fade (not affected by LR mix faders) or post fade (monitor channel will follow LR mix faders). Do this by holding the ‘Pre Fade’ key (far left side above ‘assign’) then use the select button on each channel to assign it to either Pre or Post fade. Select button light on means it is setup as Pre fade, light off is post fade (when pressing and holding the Pre Fade button). If assigning post fade as in the OP’s situation, start with the channel faders at unity and adjust from there. Most monitor mixes though are usually set as prefade.
Make sure the mix is routed out of the SQ appropriately in the ‘I/O’ screen.
Setting a particular monitor mix to mono or stereo is done in the ‘Setup’ screen, using ‘Mixer config’ and ‘Mix Stereo’ tabs.