Hi Chris, yes that's right, initially I changed only the additional one and had the same problem. Now both primary and secondary IPs are set as static and I have no issues at all. I will also remain on static IP just to be on the safe side :)
Chris, the instructions given are for Centos 6 not 7, do you think this is a problem? I've changed the additional IP to static but I don't want to change the primary server IP settings for obvious reasons...