I think your used to meraki mr nat mode.
Mx does not have that mode.
You would need to set some firewall rules yourself. For example
source: guest subnet, destination: 10.0.0.0/8,172.16.0.0/12, 192.168.0.0/16 )
&
Source: 10.0.0.0/8,172.16.0.0/12, 192.168.0.0/16 destination : guest subnet