Does the Multiplayer object require port forwarding?

You're not addressing the core problem here. Construct is heavily marketed as a tool that A) very easily allows the creation of multiplayer games for a specific set of platforms and B) doesn't work as advertised within the scope of how the tool is being marketed.

I don't understand why you're refusing to acknowledge the marketing side of Construct. Construct doesn't exist in a solely technological silo anymore. You're not just engineers talking to other engineers about engineering things. I'm a customer who bought into what you were trying to sell me, and now you're hitting me with restrictions that goes against what you were communicating in your marketing.

Whatever technological limitations there are, it needs to be clearly communicated in your marketing material. You don't get to sell me on how easy and great Construct is, take my money and then unload all these caveats on me.

I think it's reasonable to say that if you're buying a car, the state of the roads aren't the manufacturer's responsibility. They can say "drives well on roads" without having to highlight "by the way, if you drive it off-road or on a nightmarishly potholed dirt track, it might not drive quite so well", because that part is not their responsibility. The Internet is the road that web-based software drives on. I think this approach is reasonable and I'm afraid I don't have anything more to add to this particular part of the conversation.

I think it's reasonable to say that if you're marketing a car as able to drive on road type A, B, C, D, E and F and also advertise it as having radio if drivers pay a premium, then you can't leave out the fact that the radio won't always work if drivers drive on road type E.

I accepted that Construct was experimental technology back when HTML5 game design was new and you guys were focusing on creating impressive HTML5 technology stacks, but you're now charging just a few dollars less than companies like Adobe are, and I don't think it's reasonable for you to market Construct as being immensely easy to use for the creation of both mobile games and Multiplayer games, only to have Multiplayer be restricted to a certain set of mobile carriers.

I've followed your work for 11 years, and this is the first time I've ever raised a concern. I think I (and others) deserve more than a dismissive comment about how it's not your fault.

Your marketing doesn't match reality. You need to address this, and I say this with good intentions.