Wrong MOSFET transistor model? MultiSim
What you build is a source follower which acts sort of like a voltage copier, it copies the gate voltage to the source (with a Vgs drop in voltage).

What is needed for Rds to be as low as you want it is that Vgs is maximised so the source needs to be grounded.

Alternatively you can use a PMOS if the grounds need to be common.
Do note that the polarity at the gate of the PMOS switch is reversed compared to an NMOS !
Note also that the PMOS circuit is the same as the middle NMOS circuit but upside-down !







simulate this circuit &ndash; Schematic created using CircuitLab