It sounds odd, it usually should work changing the template. My initial thought-line on this issue goes like this, maybe it can be of use.
Is the iptable firewall package installed in the minimal template?
I'm thinking it may be iptables that is missing, since minimal templates can be used for offline purposes too, then iptables is probably not included like most other things that has been removed.
If iptable is not enough, then my thoughts go like this instead;
- It seems very likely to me that it is a missing package and not a missing configuration. Usually swapping templates just works as long the right packages are installed, and no configuration required. So it "seems" that it is pre-configured out-of-the-box in the installed packages, for whichever package that is missing.
- If may be that Qubes don't provide firewall functionality if the existing packages work anyway. Why fix something that ain't broke? So there is a possibility you don't need the Qubes packages to fix this. If all the relevant Qubes agent's are installed, then it's probably not this causing the issue.
- If Qubes tools are installed, networking works etc, and you got iptables installed already, then my thoughts are that it's likely missing system-config-*'s and the unavoidable full array of dependencies going with it.
- Try clone the template and essentially go berserk and not holding back, install the entire system-config- array of packages, see if networking works. If not, then either something is still missing, or firewalling has nothing to do with the system-config packages.
- If it works, then try narrow down which packages that are used for firewalling, perhaps you can reduce the amount of dependency packages being pulled if you install just the package that firewall is using.