I wouldn't feel confident enough in the quality of their WAN connection to rely upon a cloud based service. Common ports is a good idea, I'll have full control over what ports to run this stuff, so maybe it skirts their firewall rules.
I don't think it's common for DHCP leases to expire and upon renewal give you back something different, right? I guess if I actually get a stable feed and it cuts out that's something to watch for. A dropped connection would be a reason to run back.
Using an old phone at least there's no large investment in equipment needed to try this. 3D print a stand for the phone, bring an extra phone charger and I'm set.
If I end up doing this, i'll update the post after the cruise ?