Are you referring to one of the internal effects units, or to an external effects unit?
For the internal effects units, you can use the FX1 or FX2 bus (or even one of the mono mix buses) to send the desired channels to the effects unit, and switch off the channels in the main LR mix (if I understand correctly what you mean by “serial effects unit”). The effect return can then be sent to main LR (100% wet).
For an external effects unit, I think you would have to use one of the mono or stereo mix buses and corresponding outputs to send the channel mix to the effects unit, and use one of the inputs (probably a stereo input) to feed the return signal to main LR. In this case you should also switch off the channels in the main LR mix, so you hear only the effect return (100% wet).
As far as I know, Qu-16 will not let you patch insert points to physical outputs/inputs. Each connector on the back has a fixed dedicated role, similar to what an analog console would provide.
To switch a channel on or off in a mix, select the mix, then hold down Assign and press Select for the desired channel(s). The channel fader will still control any post-fader effect sends, so you can control the level into the effects units.
I hope this helps.
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