Using --ttl-inc with i-tables

I'd like to increment the TTL of routed packets going out of a tethered device so that the packets appear to originate from the router (even TTL) not the device they came from.

I've tried the following i-tables command but it is not working.

root@OpenWrt:/# iptables -t mangle -I PREROUTING -i eth2 -j TTL --ttl-inc 1
iptables v1.8.3 (legacy): unknown option "--ttl-inc"
Try `iptables -h' or 'iptables --help' for more information.

Any ideas how I can do this?