I have also faced the same issue with my Echo Spot. It is connected and showing offline. I knew that it is nothing connected with the WI-FI signal strength. My home is covered very well with Wi-Fi signal using three Apple airport Time capsules (routers) offering mesh connectivity using (WDS, not 802.11s).
I have three more voice assistants, echo, echo dot and Google Home and these are working without any issues.
The issue is caused by totally different reasons. I have bought the echo and echo dot from US a couple of years back (when these were not marketed in India) and tweaked it to recognise Indian time. The echo spot is bought from India. So with the same Amazon account I could not solve the issue. But echo spot started showing online when I created a new Amazon account using my iCloud mail id. Old ones are working with my gmail id
Now echo spot cannot talk to other echos as well as it refused to control my smart lighting. It is linked to the older echos !! So it is a pain when you have multiple echos bought from different countries to make it work together.