Yes, i see what you mean. Perhaps (and i'm sure you have thought of these already):
-hard code OPENDNS's dns servers into ROBIN as the default DNS servers, unless the administrator specifies differently on DashBoard. it's understandable that this may require a manual reflash of nodes for this change to make it to nodes that are behind such a router.
-disable ip6 on the nodes, if possible, easy, and not likely to cause a problem.
As you can't control what is used as the upstream router/dns, eliminating it as a potential cause for problems may be helpful to you in solving the root of this problem.