AST SpaceMobile (ASTS 4.69%) stock popped on Wednesday, closing the day 3% higher in response to news that Scotiabank had initiated coverage of the stock with a sector outperform (i.e., buy) rating. On Friday, for the second time in three days, the tiny cellphones-to-satellites communications company is again enjoying a much-appreciated lift -- this time, thanks to Switzerland's UBS.As of 10:30 a.m. ET, AST stock is up a solid 6%.UBS says it's time to buy AST SpaceMobile stockIn Wednesday's note, Scotiabank ...