Product/components used and version/fix level:
I am using the Ewon Flexy to connect to Cumulocity using SmartREST 2.0 over MQTT. I am working with a customer who owns the tenant so I cannot see which version.
Detailed explanation of the problem:
For ‘slow’ connections (over cellular), there is a certain payload size that will cause the MQTT broker (server) to disconnect. However, the sever keeps the TCP connection alive, but does not ack TCP. Can you tell me if there is a payload maximum, or if you have found this issue in the past? We have tested over faster ethernet based connections and this is not an issue. I have searched this forum for fixes, but did not find anything.
Error messages / full error message screenshot / log file:
new users cannot upload attachments.
Question related to a free trial, or to a production (customer) instance?
are the users doing the attachment upload via MQTT? There is a general limit of 16kb on the MQTT Payload size  in newer version of Cumulocity (old was 8kb).
Not sure if you are running into this or into any (TCP?) timeouts - I don’t full< understand the Wireshark capture here, to be honest.
The IP in your log seems to point to the public US cloud - so they should be on 10.17 by now.
 Device integration using MQTT - Cumulocity IoT Guides
Sometimes the MTU size on the device/router can cause some unexpected behaviour with protocols caused by unexpected packet drops. Historically some mobile networks have done some very strange things and undocumented things which might cause some problems with MTU values larger than 1500 bytes (including overhead).
Here is an article which goes into some of the technical details: