@BigHarryMonkey
Good day,
Thank you very much for your time and patience.
Since you had two RE450s, only one of them had this issue, the other would stay connected and keep a fixed IP address, right?
Have you tried to manually set up a static IP address on this RE450?
Or have you tried to reserve a fixed IP address on the C5400 for RE450, such as https://www.tp-link.com/support/faq/1554/
By the way, what is the hardware version of RE450? Is it V2 since we have released a new firmware for RE450 v2:
RE450(EU)_V2__200914
https://static.tp-link.com/2020/202010/20201014/RE450(V2)_200914.zip
Thanks a lot for your cooperation.