Hi and welcome to Digi Forum.
Basing on the description is not very easy to understand this behavior and where could be the issue. In general, is for sure possible to have 3 or more devices connected on the LAN and doing internet traffic. So in your specific case, we would need to know which kind of protocol is used and which amount of data is being generated. One thing that also need to be checked is the LAN itself, which kind of switch is used? Then, we would need to check the configuration and and the status of the cellular connection to see if there is any issues on that side.
This is not something we can support well via the forum, as it will require further analysis, so I would suggest you to send all details to firstname.lastname@example.org so a case will be opened for your issue.
Please also have a look at Digi Support levels options here: https://www.digi.com/support
Digi Technical Support Team