Hello   @Fabti 

You could try to set up the range extender in the normal way, then connect a network device to the extender's Wi-Fi network and see if the portal page ( a page pops up for authentication before using Wi-Fi network)  pops up or if you could enter a code to connect to Wi-Fi. 

Range extender can not bypass the portal authentication page for network devices, and they will still need to go through the portal to able to use the Wifi network. 

Range extender may have trouble connecting a Portal Wi-Fi or not compatible with a Portal Wi-Fi due to uniformed ways of portal authentication, if this does not work, you will need to use this extender with a standard Wi-Fi router, or considering the RE450 to the router via Ethernet and use it in AP mode. 

Hope this can provide any help~