I would like to note that these hosted proxies aren’t always the most reliable. When they pull the plug your app could become unusable. If you write your own proxy you’ll always have full control, but you should now how to do that off course 
Too bad not all backends properly implement CORS, these situations can give massive headaches sometimes. If you’ve solved it once though, you know what to do when you encounter this again 
And as @rapropos says, I think you should check technical support first and explain your CORS issues. Them fixing the problem would be the best solution, but I also understand that that may not be their priority (only yours and some other folks perhaps as well).