iptableslb is a tcp loadbalancer which uses iptables for the heavylifting.
Setup a jump between your FORWARD chain and the iptableslb managed one:
iptables -t filter -A FORWARD -j iptableslb-forward
and also one for the NAT prerouting:
iptables -t nat -A PREROUTING -j iptableslb-prerouting
and if nat hairpinning is wanted, also postrouting:
iptables -t nat -A POSTROUTING -j iptableslb-hairpinning
make sure those rules are appended after your firewall configs and before your "Drop everything else"-Rules