I finnaly got the answer. The problem was coming from the url used to access the POS. 

The server was running on my computer and I was accessing the POS using localhost, but the problem was that the iotbox use the address used in the address's bar to call the server and retrieve the css. So what should be used is the ip you got on the local network

Hope this help

the IoT box is not updating its time, and therefore is rejecting  ssl certificate of odoo website. 

new version of Iot box should he available this week