@Carl 

Thanks, Carl.

Unfortunately, something deeper must be going on:

The clients do not appear at all in the client list on the basic management page, not even with the altered MAC addresses. The only ones I can see are the extender itself and the devices directly connected the router. Yet, the DHCP page lists all the clients with the correct MAC addresses, rather than the ones substituted by the extender.

All I can tell from that is that the part of the router that deals with DHCP can actually query the MAC address directly from the client and make use of it, whereas the part that deals with listing the clients doesn't even know that the device is there. It's almost as if the router is treating those clients the same way as it treats the network devices that set their own IP addresses: it passes the packets through but otherwise ignores them.

Before you ask:

All addresses are allocated by the router via DHCP;

All the devices are set to obtain their IP address from the gateway;

and the extender has DHCP turned off.

And here is a curious thing: my previous Netgear R7000 router was doing the same thing. That was one of the reasons I decided that it was time to replace it after four years of mostly trouble free use.

@Carl 

I suspected that the range extender might have a hand in it, but I'm not sure what that would be, since the router seems to be allocating the IP addresses correctly.

I have double-checked the RE450's management interface. DHCP is definitely set to Off.

I have also pinged the router's address from one of the NASes, and it's all fine; the router replies within a few milliseconds.

As to why the same thing should happen with both routers, I would not be surprised if both Netgear and Tp-link use the same underlying libraries, which probably has the flaw.

@Carl 

No, still doesn't show up, I'm afraid.

I wonder if being connected via the extender's ethernet port has something to do with this. I can't connect the NAS via wireless, but I will try with a wireless device connected via the extender. Let's see how that behaves.

@Carl 

Thanks, Carl.

I have tested it. Just as you had expected, the behaviour is the same with the wired and wireless connections to the RE450 extender:

The device gets on the network, and the router allocates an IP address using the correct MAC address of the device, but the device doesn't appear in the clients lists on the front page of the management interface. Same with the Tether app. But the device is listed with the correct MAC address in the DHCP clients list in the Advanced tab.

Ironically, I bought the RE450 when a Netgear extender failed to work with my Netgear R7000 router. The RE450 worked more or less straightaway, but the same quirk happened with that router. So, I thought, "If the Tp-link extender works better with another brand router than their own extenders, using a Tp-link router should make everything work perfectly." I clearly thought wrong. On the plus side, C3150 is faster than the R7000.

If it is any help, here is my setup:

* Archer C3150 router with wired and wireless clients;

* RE450 extender. Connects to the router with the 2.4 GHz link and serves clients on the 5 GHz link;

* Two Synology NAS boxes connect to the wired port of the RE450 through a Tp-link TL-SG108S switch;

* The router allocates all IP addresses. Most clients get fixed addresses through DHCP address reservation; guests get allocated addresses from a small pool.

The quirky behaviour is the same whether a NAS uses the RE450's ethernet port directly, the Tp-link switch or a Netgear GS108 switch.

@Carl 

Thank you very much for chasing this, Carl. Very kind of you.