Digi WR21 & Digi Transport WAN VPN loses communication in the ethernet port.

Digi WR21 & Digi Transport WAN VPN loses communication in the ethernet port.

I have a number of systems in the field (60+). Only 10+ of this sites has the following behavior (both models).

I have devices connected to the same router by Ethernet CAT 6 cable without problems.

You can use normally the port of the router with another device (like laptop), but this does not respond with my controller card (c-power 5200).
A power cycle of the router fixes the issue. but it may take two days or two weeks to lose connection with my controller again.

Some advice about this issue?

Thansk in andvance for your support