@tanpanjang
Hi,
Thanks for your patience.
Since the Archer C5400 is still working on the DHCP router, if just using the Deco to extend the Wi-Fi range, access point mode would be much recommended on the Deco in case of double NAT issue;
When Deco works on the access point mode, there are little differences between each model so Deco M4 or M5 would be more competitive on price and performance;
But if you still want to use the Deco as the wireless router, Deco M9 plus, Deco X20, and X60 would be more recommended;
PS: the difference between router mode and access point mode:
https://www.tp-link.com/en/support/faq/2423/
Thank you very much and wait for your reply.