Try this if you have frequent disconnection/connected but no wifi on your RE450 range extender
Try this https://www.tp-link.com/us/support/faq/943/
if you have frequent disconnection or connect but no wifi on your RE450 wireless range extender.
The methodology is to connect the extender to the main router through (2.4GHz) and the extender to your devices through (5GHz) instead of using them interchangeably which might be the reason that causes all these problems.
WEB GUI Login
However WEB GUI login will be different from the one stated. You will have to use the range extender wifi to log in (try it whenever there is connection via your range extender) then it will show you a page that requires only a password. Instead of using admin as the password, you will have to use the password that you initially set up for your wifi range extender (not the main router password).
I tried this after my range extender had those problems the second day after I used it. It really worked and till now there wasn't any more problems.