Firewall rules for transmission
Solution 1:
So as I explained in the comments above, it was a typo I made Lol ... I had my INPUT udp port as 51513 instead of 51413 ...
But, just in case anyone wants to know, these are the rules that I have used to allow Transmission:
iptables -A INPUT -m state --state RELATED,ESTABLISHED -p udp --dport 51413 -j ACCEPT
iptables -A OUTPUT -p udp --sport 51413 -j ACCEPT
Thanks everyone for their input