I think we can forget about ATC Brokers (Cayman Islands).
On closer inspection, ATC Brokers is a dead-end for U.S. residents (other than high-net-worth individuals). In other words, for almost everyone on this thread, ATC Brokers is nothing more than an IB (reportedly for Oanda). If you want to open an account with Oanda, do it directly – you don’t need to go through ATC Brokers.
ATC Brokers is a member of the U.S. National Futures Assoc. (NFA), the sock-puppet of the CFTC.
ATC Brokers is not going to incur the wrath of the CFTC/NFA Nanny State by trying to onboard U.S. residents who do not qualify as professional clients.
Also, it’s most likely that the IB contract between ATC and Oanda stipulates that ATC will not accept non-professional U.S. clients, but will refer them directly to Oanda. If that is the case, then ATC has no incentive to onboard ordinary U.S. traders directly.
Here is the ATC document which defines the various classes of client –
Regarding the requirements for re-classification as a professional trader (in order to open an ECN account directly with ATC, rather than being dumped off to Oanda), here are the operative paragraphs from the document linked above (click on the image to enlarge it) –