ez pz:

#!/usr/sbin/nft -f
table inet filter {
    chain input {
        type filter hook input priority raw; policy accept;
        iif "lo" accept
        ct state established,related accept
        iif "enp1s0" udp dport 51820 accept
        iif "enp1s0" drop
    }

    chain forward {
        type filter hook forward priority raw; policy accept;
        iif "lo" accept
        ct state established,related accept
        iif "enp1s0" udp dport 51820 accept
        iif "enp1s0" drop
    }

    chain output {
        type filter hook output priority raw; policy accept;
    }
}