Did this solve the problem?  I installed a RE650 Extender to connect to my AC5400 yesterday.  I did set a hard coded IP address to the Extender.  All wireless devices, whether they were connected to the Extender or the AC5400 router lost connectivity to the Internet.  I could log onto the router from my PC via my wifi connection and the router showed full connectivity, but no Internet access.   All devices were restored once I unplugged the RE650.  This impacted all wifi devices throughout the house.  All wired devices continued to work fine.  Since I did set a hard coded IP address to the Extender, I am not sure what to try now.

@Mesa I just had the same thing yesterday. I plugged it in, followed the instructions and it worked for 15 minutes but then lost connection to the host internet and somehow 3 separate times turned my fios wifi signal off. I dont know the proper terminology for what happened but the router lights turned off and I had to refresh it at the router 3 separate times. That happened 3 times, plugging the RE650 into two different outlets. I then unplugged RE650 and my router has been fine. I'd love to know a solution for this problem or I'll need to be returning this wifi extender.

@hgraeber I have been testing the RE650 with no success.  I tried changing it to Access Point mode.  Now the router does not seem to be impacted, but any device connected to the RE650 does not have Internet access.  I also tried changing the DHCP pool.  Auto configured the same pool the router was using.  I have configured the pool to use 200-220, with no luck.  I also turned DHCP off hoping the RE650 box would pass the DHCP request to the router.  It does not do this.  I had to hard code my PC to gain access to the RE650.  I am not sure what to do at this point.  I bought the RE650 assuming it would work well with the TP-Link AC5400 router, but that does not seem to be the case.  Hopefully, someone from TP-Link support (working from home) will see these posts and respond.

Update.  I changed the Extender back to Extender mode, from access point.  Then I changed the network connection back to "Obtain an IP address automatically".  My DHCP is still on set to a range of .200-.220.  I did a release/renew of my IP address.  I was assigned an IP address in the range the router uses, .100-.199 and the default gateway is set to the router, not the extender.  So the PC is now getting it's DHCP information from the router.  The Extender still shows the status of the Internet as Unknown.  My PC, however, is now accessing the Internet fine being connected to the Ethernet port on the RE650.  At least is is working for at least the time being.  I will test the extender throughout the day to see if this configuration keeps a stable link.   

Hi,

I just got Wifi Extender RE650, I tried to connect the wifi extender to the wifi router using internet cable. The problem is there's no signal at all. I already set it first and its all okay without the cable. but when i tried to connect it via internet it looses the signal.

Kindly advise on what to do.

Thank you