Old Wireless Keyboard and Trackpad with New(er) iMac.

I have arrived home with a recently purchased iMac, Circa 2015, to replace a non functional 2010 model. The seller reset the 2015 and on start up it arrives at a "Welcome" setup page, but nothing I have done has the 2015 recognizing the availability of either the wireless apple keyboard or trackpad.

Basically all I have tried is to put both wireless devices into discovery mode before starting the 2015 iMac.

I now assume I will need, at a minimum a either a wireless keyboard or mouse to get the 2015 set up before connecting the wireless devices. But I am hoping to avoid that.