Yes. Use version 2.0 of the WeatherCat client. Install (copy) it on your MacBook. If your port forwarding is setup correctly, the Client will pull up your WeatherCat data on your MacBook.
I'm not sure about running the old version of the WeatherCat iOS app on your setup. I deleted mine when I upgraded to the RCP version.
However, I do have an idea (which doesn't happen very often). On your router, make sure you are forwarding ports 49250-49255 to the internal IP address of your Mac Mini. It is much simpler if you assign an internal static IP address to your Mini so that it doesn't have the ability to grab a new IP address every time you reboot it. Then in the iOS app, enter your DYNDNS address for Internet 1. It doesn't really matter whether you enter it in Internet 1 or 2 but leave the Trixology address in one of them and put your's in the other one. You want Trixology so you can test to make sure the app works correctly. If it does, that narrows your issue down to something on your end.
I suspect that if your Mini does not have a static internal address, the reason your iOS app isn't working is because you rebooted your Mini at some point and it grabbed a different IP address from your router. So either give it a static internal IP by assigning the MAC address of the Mini to one of the addresses in your internal range, or find out what internal IP address the Mini is currently using, and forward the ports to that address and it should work, until you have to reboot.
If you are not using a static IP address on your Mini, it won't necessarily grab a new one on every reboot, but it can, if something else (either mobile or your Mac Book) came along and grabbed the old IP address of the Mini whilst it was turned off, or was rebooting.
Hope this helps.