I also have a Tp-link RE450 extender (last firmware may-2016) and ... everything works perfectly.
As I said in other posts (and also to Merlin) I do not like the way in which the DHCP Server Asus manages the Extender, but that's another story..
Your problem probably is this firmware with 87U (I think).
What's your router? I also belive the problem is the 87U...
The DHCP in Server Asus can be the problme?