This is a modification for the new version of OpenWrt, we are not responsible for maintaining LuCI.
For now, you can only modify it via SSH.

For All Interfaces:

mkdir -p /usr/share/nftables.d/chain-pre/mangle_postrouting/
echo "ip ttl set 65" >  /usr/share/nftables.d/chain-pre/mangle_postrouting/01-set-tt
l.nft
fw4 reload

For one Interface (such as eth0):

mkdir -p /usr/share/nftables.d/chain-pre/mangle_postrouting/
echo " oifname eth0 ip ttl set 65" >  /usr/share/nftables.d/chain-pre/mangle_postrouting/01-set-tt
l.nft
fw4 reload

It is optimized for tighter Block Non-VPN Traffic in GlobaL Options of VPN Client.