We have 4 of these switches, all of which are having the same problem. When we connect more than 9 devices, the switch is not forwarding on packets. This means that we cannot use the passthrough capability on our SIP phones.

This is our configuration

Ports 1-6: 6 x Cisco 7975 phones + PC using passthrough (100Mb/s link)

Port 7: 1 UPS (100Mb/s link)

Port 8: Trunk to upstream switch - Cisco SG350X-24 (1000Mb/s link)

We're not using any VLANs and the power is well within budget (30W).

When I look at the MAC address table on the upstream Cisco switch, I see 10 MACs on the port that is connected to each TP-Link switch.

Some of the TP-Link switches can't even be connected to, because the 10 MAC addresses it's provided to the upstream switch are for the connected devices, so ARP requests aren't being responded to. In these cases, 10 of the devices have connectivity to the network.

On the switches that I can connect to, only 9 of the devices have connectivity to the network. When I look at the MAC address table on the TP-Link switch, I can see the addresses of all 13 devices on Ports 1-7, and a complete MAC address table on Port 8, but the upstream switch can only see 9 of the MACs (plus the MAC of the TP-Link switch).

So, the question is, is there a device limit for these (I can't believe that this is the case).

Edit: In addition, I connected a Wireless Access point to one of the ports, and this causes the same problem. Once 8 WiFi devices are connected to the AP, no more can connect. This is because it reaches the 10 port limit (1 Switch + 1 AP + 8 devices = 10). So it seems that there is a hard limit of 9 devices and that this switch is rendered useless if you intend on connecting a Wireless Access Point to it...