...
/interface/wireless/access-list add authentication=yes forwarding=yes interface=wlan2 signal-range=-55..0
Then the connection is not matched to any the ACL rule and , but if signal drops to -70..-80, client will not be disconnected.
Please note that if "default-authentication=yes" is set on the wireless interface, clients will be able to join even if there are no matching access-list entries.
To make it work correctly it is required that client is matched by any of ACL rules.
If we modify ACL rules in the previous example to:
/interface/wireless/access-list
add interface=wlan2 signal-range=-55..0
add authentication=no forwarding=no interface=wlan2 signal-range=-120..-56
...