Here, see proof,
Does every stand has it’s own international area?
Actually you can build one international zone containing all stands.
If I see it correct, International Zone 3 works well. It’s setted up how it’s supposed to be. The first floor will also be International Zone 3.
The one at the bottom doesn’t, as the passport checks aren’t correctly zones. (may a reload helps)
Here a simplified setup:
Also keep an eye on the zone numbers and if all have green symbols for entry and exit.
If you change floors, make sure to have the other floor marked with the correct zone as well.
If you use only elevators, they sometimes do stupid and zones do not merge. Best is to have at least on staircase or escalator. And if it still doesn’t merge, reload the savegame.
I noticed that Passengers aren’t going, just Checking Out from the passport check-in and not going out. And with that, here is an update.
I still need to add more Passport check-ins.
Question: How to make it so that the temp In-Game should be under 0?
Does the zone say that it has 6 outgoing passport checks?
And as written above, I think the issue is, that you are only using elevators which do not merge zones correct. May build a staircase in the non international part.
Otherwise, upload your savegame to the Steam Workshop and link it here. It’s not easy to find out what is the problem when we only see small parts of your airport and not the entire setup. 
Regarding the temperature: This depends where your airport is located. If your airport is in warmer regions, there is no chance to get it lower than 0 degrees. In those cases, buiding of de-iceing objects is blocked.
In cold areas, it’s very likely and sometimes the entire year below 0.
In other areas, cold temperatures may only be reached during winter or nights.
What happens if I post that in the steam workshop and post the link here? Whatever you build in you’re device, will it update for me also?
What if I use the Dev Panel and put it to snow and temp in freezing?
I added 1 Escalator near the Elevator. I deleted the Elevators.
Is it now working better?
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Great.
By the way:
For departures you do not need that many passport checks as the passengers drop in in less numbers (due to the previous security screening)
For arrivals it makes more sense to provide a bigger number of passport checks.
Alright!