Use the official commands written on the OpenWrt Wiki:
Or change the TTL and IPv6 hop limit in nftables with these commands:
# Modify TTL and IPv6 hop limit for egress traffic on written devices
WAN_TTL="64"
WAN_HLIM="64"
WAN_DEV="eth1, ppp0, ppp1, usb0, usb1, wwan0, wwan1"
# Add TTL, IPv6 hop limit and firewall rules for egress traffic
cat << EOF > /etc/nftables.d/ttl_and_hlim.sh
nft add rule inet fw4 mangle_postrouting oifname { $WAN_DEV } ip ttl set $WAN_TTL
nft add rule inet fw4 mangle_postrouting oifname { $WAN_DEV } ip6 hoplimit set $WAN_HLIM
EOF
uci -q del firewall.ttl_and_hlim
uci set firewall.ttl_and_hlim="include"
uci set firewall.ttl_and_hlim.path="/etc/nftables.d/ttl_and_hlim.sh"
# Saving modified values of firewall
uci commit firewall
service firewall restart
# Check the rules of the chain "mangle_postrouting"
nft list chain inet fw4 mangle_postrouting