There seems to be no evidence that he was taught radicalism while abroad. The article goes in depth analyzing his YouTube, his activities, and various experts say it seems to point to the fact that TT self-radicalized. There is no evidence from his activities there that he had joined a group, received training, or was part of anything bigger.
So when Russia calls and says, hey, this guy visited XYZ mosque, may e you should check him out, and we do, and find no evidence that he is a danger, what then should we do? D you want to see the US become more like Russia, and crack down in an authoritarian manner against anyone who visits certain houses of worship, or certain groups? What?