Back To My Mac setup and use.

I am tech support for my 84 year old mother. She needed a new computer so I installed an iMac for her today. I set up her iCloud and Apple Id, enabled Back To My Mac (abbreviation: Back). By the way, her new computer runs Yosemite. She lives in a residence where WIFI is provided, no access to routers etc. When I got home I tried to connect but could not. I made a special account on my MacBook Pro running El Capitan, signed In with her Apple ID. Went to finder and, you guessed it, no shared devices. I had set her computer to wake up as well.

What have I done wrong?

I did read about base stations, routers etc.

Any help would be appreciated. Prior to getting her an iMac, I would log into her PC with Logmein which is still an option but I would prefer Back.

Thank you!