airport extreme

so should probably check w/cable provider to verify they're sending 400Mbs

The cable company will always say that everything looks good. Connect your Mac directly to the modem using a good CAT5e or CAT6 Ethernet cable to see exactly what is happening. You need to see 300-350+ Mbps, before we can start to troubleshoot the Airport Extreme.

A seven year old AirPort Extreme should still do 275-300+ Mbps if it is operating well when you do recheck the Ethernet ports on the AirPort.

A bad Ethernet connection will still allow an Internet connection, but speeds will be limited to 100 Mbps even though they are actually much higher. Make sure that you have good cabling.

What happens when you test the speed of the Internet connection on the AirPort Extreme by connecting your Mac to one of the LAN <--> ports on the AirPort Extreme using an Ethernet cable connection?

Does the extender connect using a wired Ethernet cable connection back to the AirPort Extreme, or does the extender connect using wireless? What model number of extender do you have?

In general, wireless routers designed for home use will go about 5 years, on average. That being the case, your AirPort is due for replacement, but if it performing well now, there is no rush.

You certainly won't achieve 400Mbps on wireless via the 7year old Extreme.

Nor will N wireless clients of course.. so it makes no sense to upgrade the airport unless you upgrade clients as well.

Everything needs AC wireless now to reach actual 400Mbps throughput.. and that will be on a good day.. standing about two paces from the airport.

Ethernet should get you full speed or near to.. again the older airport extreme processor will only just reach throughput that high. New one is not much better because it is now 5 year old design.