I have just tried again, with RancherOS 4.2 and your addition to the cloud-config.yml, and it now works! No more hanging when trying to download the images. Not sure if this is v4.2 or the extra option, but whichever it is, it solved the problem.
I don’t think we have any 172.18 around the place getting in the way, but of course it is possible.
Thanks for thepointer