@Smithnet 

Hi, welcome to the community.

Since you mentioned the DDNS stopped working after changing the main Deco, I did see this feedback before: 

https://community.tp-link.com/en/home/forum/topic/604478

https://community.tp-link.com/en/home/forum/topic/274402?replyId=1061648

And I wish you could help me check some details:

1. I see you mentioned that you have tried to change a new DDNS domain but it did not work, did you get the same error on the Deco APP "Failed to save your settings - please try again later"?

2. After setting Deco X50 as the main Deco, when you go to the DDNS, will you still see the previous domain under DDNS?

Thank you very much.

Wait for your reply.

Best regards.