Weird Airport networking in Argentina

The place is in Argentina.

I will paraphrase because I´m no network guru, but I kinda get some stuff.

Here´s the map so far:

Router A sends a packet with tag A to Switch C1

Switch C1 tags the packet A with an additional tag, so it becomes A+C, right?

Packet now is tag A plus C so it travels through the network C

Network C (with the +C tag, the packet A moves inside the network C)

Switch C2 receives packet A+C, and removes tag C, so it has only tag A, right?

Packet now is tag A minus C

Router B receives the packet with tag A-C only, meaning just tag A, correct?

In theory, this is what I think is should happen, right?

The issue is, the admin of Network C says that Switch C1 is in access mode, and Router A must be ALSO in access mode too.

What do they mean by that? because in paper, the topology seems to be correct but the admin for Network C must have some security features that are blocking or discarding the packet, because it does not reach Router B, and Router B cannot reach Router A. And the only thing they say is that the router has to be in access mode if not it won´t work.

I can show you part of the config of router A, since actually it does not have anything else:

interface GigabitEthernet0/0.756

description --- To redacted - Gi2/3/0.756 - 2000K

encapsulation dot1Q 756 native

ip address 10.10.10.1 255.255.255.252

Then it got changed to

interface GigabitEthernet0/0.756

description --- To redacted - Gi2/3/0.756 - 2000K

encapsulation dot1Q 756

ip address 10.10.10.1 255.255.255.252

Neither worked in the end.

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The question is: what the hell does the airport admin mean with ACCESS MODE??? they would not say anything else.