Got my Siren, but rejected by URSSAF…

I have some questions around registering my new business and becoming an ME. I’ve been trawling the forum but haven’t yet found the answer to my problem. I’ll try to summarise:

• UK national, arrived in December, applied for a carte de séjour (as a self-employed person), and applied to start a micro-entreprise via the URSSAF portal. I used ‘4799A Venta à domicile’ for my APE, I sell used photographic equipment on line (via Ebay). Starting trading in anticipation of receiving the credentials.
• quickly got a letter from Insee with a Siren and Siret number (start date 28/12/20), so I thought all was well.
• later received a letter from URSSAF rejecting me from ME status as I’m not registered with the RCS or RSAC. It says I fall under the régime général des salariés rather than the Sécu.

I had followed the URSSAF path because on the Insee page where it helps you to identify the relevant CFE it says the following under the URSSAF section:  “4. Pour un vendeur à domicile, exerçant son activité de manière indépendante, non inscrit au RCS ou au registre spécial des agents commerciaux.”

This seemed to match my situation exactly, and I thought it mean I didn’t need to register with the RCS. I emailed URSSAF to follow up but they weren’t willing to discuss it. I can read and write French well enough to understand and communicate, but at this stage phone calls don’t really work. So now I’m lost!

Was I on the right path with URSSAF, and just need to register with the RCS and then re-apply to URSSAF? When I look at registering with the RCS it seems aimed at bigger commerçants and asks how I want to pay the TVA etc. I’m intending not to be TVA registered. Have I picked the wrong CFE?

It’s not even clear if I’m currently allowed to trade or not, and how I should be paying taxes. Also I’m worried that if I have to start from scratch, my new business start date will be post-Brexit and therefore I’ll be rejected.

Sorry for the long message, I hope you can help.

Matthew