pretty simple with tc command
login to router with ssh/telnet

where wl0.1 is the ssid of your guest network.
will vary if you have configured more than one

to limit to 5mbit until you reboot

tc qdisc add dev wl0.1 handle 1: root htb default 11

tc class add dev wl0.1 parent 1: classid 1:1 htb rate 5mbit

tc class add dev wl0.1 parent 1:1 classid 1:11 htb rate 5mbit

to remove the settings use
tc qdisc del dev wl0.1 root

to limit to 5mbit permanently

echo tc qdisc add dev wl0.1 handle 1: root htb default 11 >> /jffs/scripts/wan-start

echo tc class add dev wl0.1 parent 1: classid 1:1 htb rate 5mbit >> /jffs/scripts/wan-start

echo tc class add dev wl0.1 parent 1:1 classid 1:11 htb rate 5mbit >> /jffs/scripts/wan-start

Note:

Only downstream, i have to figure out upstream :(
see this SNB post about upstream

You might need to enable jffs and add them to /jffs/scripts/init-start to be presistent

Interface Reference

2.4 Ghz Radio
wl0 = Private Network
wl0.1 ---> Guest Network 1
wl0.2 ---> Guest Network 2
wl0.3 ---> Guest Network 3

5 Ghz Radio
wl1 = Private Network
wl1.1 ---> Guest Network 1
wl1.2 ---> Guest Network 2
wl1.3 ---> Guest Network 3

more info about tc
http://mark.koli.ch/slowdown-throttle-bandwidth-linux-network-interface

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