Well, we don't really know it was "his" phone.
The reporting is: "A mobile phone traced to President Donald Trump’s former lawyer and “fixer” Michael Cohen"
This suggests, to me, that its not his primary phone, and the connection to Cohen could be as tenuous as he pays the bill.
I think people are jumping the gun a bit here - and finding that 1+1 = 3
And, for all we know, Steele had access to similar data when he drew the conclusion that Cohen was in Prague.
There might be something with this - but based on the information released, it is premature, at best, to suggest it means Cohen, himself, was in Prague.