Thanks so much. I finally got it work using namecheap. DNSimple looks good, would most definitely be trying it out.
The issues were multiple. The DNS turned out to be not the problem at all 
The major issues were as a result of oversight. I left out some code (require blah blah) required for apartment to work properly and then to worsen it, I was using the heroku open command from the terminal to open the app. That goes straight to the ‘subdomain’.herokuapp.com and not the www.customdomain.com so heroku was having issues with the sub sub domain. Finally figured that out using the heroku logs -t command.
Thanks for the help too, christoomey. Cheers!