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I’m confused: is there a SmartThings aspect to this question?
This forum is for people who are using the Samsung SmartThings ™ home automation platform, so all questions and answers are assumed to be in that context. But I can’t tell from your post what you were trying to pair to. Are you using a Samsung Wi-Fi mesh smartthings router? Or you can’t add it to a SmartThings account? Or?
There is an official tuya integration with smartthings, but it doesn’t work with the ewelink app, just with the smart life app. And that’s not Sonoff.
And many people in this community using sonoff devices flash the firmware so that they can be connected directly to a smartthings account without going through another cloud, but then you wouldn’t be using the ewelink app either.
And although the smartthings hub has a Bluetooth radio, at the current time it is inactive so there are no Bluetooth devices that can be added to a smartthings hub.
So I’m confused. 
This is a very busy forum, so it often comes up near the top in general home automation searches, but it is not a general forum.
If by chance you were looking for an answer to a problem that is not occurring on the smartthings platform, reddit has a subforum on home automation which is brand agnostic. So you might try there.
If instead there is a SmartThings component to your question, we need a few more details before we can help. 