@jphicks 

Since AX5400 deosn\t have a range extender mode (only ruter mode and AP mode) you could buy a lower end modem (that supports 1000mbs), plug it onto the AX5400 and buy an additional AX1800 range extender.

Another option could be to buy a modem/router (that supports 1000mbs) and set the AX5400 in AP mode ()this requires to wire AX5400 to your modem) and connect the AX1800 RE to the AX5400 or your modem.router.

Keep in mind that to use AX5400 and AX1800 in Easymesh you have to use AX5400 in router mode, i. e. first option.

Hi@jphicks,

Thank you for contacting our community.

Could you double confirm what the model number of the router is? Archer AXE5400 or Archer AXE75?

Please confirm with the new internet service provider whether the ISP NBN box is connected to the home modem via Ethernet cable or DSL cable?

If it is Ethernet cable, then you can use Archer AXE5400 as the network input device without purchasing a new modem.
If it is DSL cable, then you also need to consider resetting the old modem or purchasing a new modem as the network input device.

For Archer AXE5400, it is a wireless router, and the main working modes are router mode and access point mode, both of which require Ethernet cable to obtain the network.

If it is Archer AXE75, it is also a device compatible with Easymesh technology, so you can consider purchasing a modem that is also compatible with Easymesh (when the network output of the NBN box is through DSL cable), so that you can connect the modem to Archer AXE75 wirelessly.

Or, when the network output of the NBN box is Ethernet cable, you can purchase extenders that are also compatible with Easymesh to extend the network range.

The following is the list of TP-Link EasyMesh Devices for Home

Best Regards~