I found this information at https://forums.centos.org/viewtopic.php?t=76822

Short version  –

My workstation using tftp cant retrieve a file from my tftp server. Stop firewalld and it works.

Solution:

firewall-cmd --direct --permanent --add-rule ipv4 raw OUTPUT 0 -j CT -p udp -m udp --dport 69 --helper tftp

Then you execute

firewall-cmd --complete-reload
firewall-cmd --zone=public --list-all
firewall-cmd --direct --get-all-rules

Tried the tftp transfer… and it works!

Additional sources for information: Command Source: https://beaveryoga.wordpress.com/2016/12/10/iptables-and-tftp-howto/