Hello @blackboard.grey 

After a successful factory reset, extender will use default SSID (TP-Link)Range Extender XXX.

I suggest factory reset again and set it up again, connect to 5Ghz of new wifi router if new router is a dual band router.  
RESET: With the extender powered on, use a pin to press the RESET button until most LEDs flash once. 
Later please follow the instrucions here to set up range extender :
（How to configure Range Extender via Tether APP）
（How to configure my range extender via web GUI to extend an existing wireless network's coverage）