Hi @TheoMer. Thanks for being part of GoDaddy Community! It looks like you have your CNAME pointed toward a Heroku site. Since the error mentions an IP address, my guess would be that there is an issue with the way the URL you're specifying in the CNAME resolves. I'm not seeing that error currently, though I am seeing an SSL error when visiting HTTPS://frontend.theomer.co.uk. That would probably be due to no SSL certificate being installed for your URL on the destination server. I'd review both situations with Heroku to see how you can resolve them.
JesseW - GoDaddy | Community Manager | 24/7 support available at x.co/247support | Remember to choose a solution and give kudos.