Please can you post your full S3 cgminer conf file? (you can exclude the pools if you want), then please can you confirm your LAN network is on 192.168.20.0?
the IP of the S3 is 192.168.20.98 as is the rest of the network is on 192.168.20.xxx
Please try to remove the trailing 0 put it as:option api_allow 'W:127.0.0.1,W:192.168.20/24'
EDIT:
Sorry try this instead:
1- Edit /etc/config/cgminer
2- Modify the api_allow argument to read option api_allow 'W:127.0.0.1,W:192.168.20.1/24'
3- Edit /etc/init.d/cgminer
4- Locate the line beginning with PARAMS= and change it accordingly: PARAMS="$AOPTIONS $POOL1 $POOL2 $POOL3 $_pb $_ow $_bec --api-listen --api-allow $_aa --bitmain-checkn2diff --bitmain-hwerror --queue 4096"
5- Restart the S3 (e.g. using sudo reboot)