I would make sure that you have the latest firmware loaded on all devices and if yes then there must be some issue with the network interface --- the question becomes which device?
The way that I would approach this is make sure that each network interface is properly configured [full duplex] --- the R6300 LAN ports are full duplex --- and then try other network devices and see if all of they react the same way --- if they do then I would log a bug report with Netgear.
Netgear are using a unique Broadcom LAN chip that has some "smarts or intelligence" and that may be causing the issue with older technology.--
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