there is a known issue with connecting v3 hubs…
Investigating
Some North American and European users may currently be unable to add the SmartThings Hub 2018 model, receiving the error “Hub not found.” We are currently investigating and will provide updates as available.
Posted 9 days ago. Jun 11, 2019 - 15:01 EDT
there is also another thread where you may want to read and post your experience…
and contact ST support if you haven’t already.