Same issue here, with 2 Echo devices showing offline in my app, and same root cause I think: some of the devices are linked to my account but on another marketplace (FR).
I tried accessing alexa.amazon.fr, but I keep being redirected to alexa.amazon.com where these 2 devices appear offline.
No luck with either the US or FR version of the iOS Alexa app.
Have you been able to solve this issue?
Can you force an Echo Show and an Echo Spot to connect to the US account when registering them?
The issue might be that I'm using the same email address for my account on the FR and US websites. Creating a new account for the US marketplace with a different email address is not a solution as I don't want to repay for Prime / Music Unlimited.