Glad that this part is solved The lan network and br-lan should not need to be modified for VPN related things.
Again, I cannot understand how changing the wifi networks with respect to the lan would suddently make the VPN work (and I highly doubt that it did actually have anything to do with it working if it ever did), since fundamentally this isn't a change required for VPN access in any normal situation.
You should consult with the people who provided the script and/or maintain this VPN/proxy service for help regarding how this is supposed to work and how you can verify that it is actually working.
This is a JSON string. But you'll have to ask the people who provided this stuff for help.
Meanwhile, based on your original subject line (about both wifi bands not working at the same time), I would say that your problem is solved.
If your problem is solved, please consider marking this topic as [Solved]. See How to mark a topic as [Solved] for a short how-to.
(Obviously you may still have questions about the VPN/Proxy, but I would suggest that it may be best answered in a new thread that focuses specifically on the VPN/Proxy issue. This may or may not really be an OpenWrt issue, though, and may require help from the originators of that json data).