DDoS attack on port 53

RouterOS General
1

i have receive 3-4Mbps(limited internet , cant be more) ddos attack form 15-16 ip on port 53
what can i do

2 
/ip firewall filter

add chain=input protocol=udp dst-port=53 in-interface=internet2 action=drop

And delete all the existing connections to port 53 from internet.