Hello,
Two ways to do that, if you want to allow a small number of devices or you want to prevent a small set.
To allow a small number of devices and prevent all the others, go to Network->Firewall->Custom rules and add:
iptables -A FORWARD -s 192.168.1.x,192.168.1.y,... -o qmimux0 -j ACCEPT
iptables -A FORWARD -s 192.168.1.0/24 -o qmimux0 -j DROP
Or if you prefer to write the reverse, explicitly forbid some devices to use the mobile interface:
iptables -A FORWARD -s 192.168.1.x,192.168.1.y,... -o qmimux0 -j DROP
Replace 192.168.1 by your local lan network of course.
Regards,