try this setting
without touching mtu
iptables -t mangle -A POSTROUTING -s 192.168.1.8/29 -j DSCP --set-dscp-class CS1
iptables -t mangle -A POSTROUTING -d 192.168.1.8/29 -j DSCP --set-dscp-class CS1
iptables -t mangle -A PREROUTING -s 192.168.1.8/29 -j DSCP --set-dscp-class CS1
iptables -t mangle -A PREROUTING -d 192.168.1.8/29 -j DSCP --set-dscp-class CS1
iptables -t mangle -A POSTROUTING -j DSCP --set-dscp-class EF/101110 -s 192.168.1.124
iptables -t mangle -A POSTROUTING -j DSCP --set-dscp-class EF/101110 -d 192.168.1.124
iptables -t mangle -A PREROUTING -j DSCP --set-dscp-class EF/101110 -s 192.168.1.124
iptables -t mangle -N qos_plus_up
iptables -t mangle -A PREROUTING -j qos_plus_up
iptables -t mangle -A INPUT -i pppoe-wan -j qos_plus_up
iptables -t mangle -A FORWARD -i pppoe-wan -j qos_plus_up
iptables -t mangle -A qos_plus_up -p tcp -m tcp --tcp-flags PSH PSH -m length --length :60 -j MARK --set-mark 0x2
iptables -t mangle -A qos_plus_up -p tcp -m tcp --tcp-flags ACK ACK -m length --length :60 -j MARK --set-mark 0x2
iptables -t mangle -A qos_plus_up -p tcp -m tcp --tcp-flags SYN SYN -m length --length :60 -j MARK --set-mark 0x2
iptables -t mangle -A qos_plus_up -p tcp -m tcp --tcp-flags RST RST -m length --length :60 -j MARK --set-mark 0x2
iptables -t mangle -A qos_plus_up -p tcp -m tcp --tcp-flags FIN FIN -m length --length :60 -j MARK --set-mark 0x2
iptables -t mangle -N qos_plus_down
iptables -t mangle -A POSTROUTING -o pppoe-wan -j qos_plus_down
iptables -t mangle -A qos_plus_down -p tcp -m tcp --tcp-flags PSH PSH -m length --length :60 -j MARK --set-mark 0x200
iptables -t mangle -A qos_plus_down -p tcp -m tcp --tcp-flags ACK ACK -m length --length :60 -j MARK --set-mark 0x200
iptables -t mangle -A qos_plus_down -p tcp -m tcp --tcp-flags SYN SYN -m length --length :60 -j MARK --set-mark 0x200
iptables -t mangle -A qos_plus_down -p tcp -m tcp --tcp-flags RST RST -m length --length :60 -j MARK --set-mark 0x200
iptables -t mangle -A qos_plus_down -p tcp -m tcp --tcp-flags FIN FIN -m length --length :60 -j MARK --set-mark 0x200
if you analyze with SG TCP / IP Analyzer from
https://www.speedguide.net/ With the IP assigned to EF, you will know that DSCP is working with EF and if you analyze with Mobile or another separate PC, it will result in DSCP CS1