Sorry to be answering this by myself, but perhaps this will help somebody someday: Actually, it is working as expected right now! Seems like the cellular connection was kinda flaky with my first tests. I simply disabled No-NAT for WAN2, made sure I had a stable LTE signal and everything worked flawlessly. Strangely enough, it doesn't seem to work when WAN1 is set to No-NAT. I know you guys aren't too happy with the way No-NAT is implemented currently but this could be something to remember when developing this further. Thanks a million @Raj66 for the hint!
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