Внешний (WAN) порт - eth0, внутрений (LAN) порт - eth1.

nftables.conf

table ip nat {
    chain forward {
        type filter hook forward priority filter; policy drop;
        ct state establised,related accept
        iifname "eth1" jump forward_lan
        ct status dnat accept
    }   
 
    chain forward_lan {
        oifname "eth0" accept
        oifname "eth1" accept
    }   
 
    chain preroute {
        type nat hook prerouting priority dstnat;
        iifname "eth0" tcp dport 8080 dnat ip to 192.168.0.5:8080
    }   
 
    chain postroute {
        type nat hook postroute priority srcnat;
        iifname "eth1" oifname "eth0" masquerade
    }   
}