I would like to explain my suggestion to add a length of cable between the A3 and the “enterprise class” switch. I have observed that some of the commercial switches that are designed to comfortably drive 1000M through 100m of cable are producing a rather powerful transmit signal. If you connect that to an Ethernet interface that is designed for “hobby” type application, the little transformers in the adapter can get saturated and make garbage out of the signal that the micro sees. The conclusion would be that this is not a valid network. It might barely work, if you force the connection to 10M because then you have some nanoseconds for the transformers to come out of saturation. But not for all data strings. Inserting a length of lossy cable between the switch and the A3 would ameliorate this particular problem (if it is the problem).

Ken