A potential megadeal between Chinese wireless carriers could give a boost to two companies half a world away: T-Mobile US Inc. and Sprint Corp. China United Network Communications Group Co. is said to be exploring a merger with China Telecommunications Corp., a transaction that would unite the country’s second- and third-largest carriers. The Chinese deal would be part of that country’s push to develop advanced 5G network technology, raising concerns that the U.S. could fall behind in that area — a vulnerability that T-Mobile hopes to address with a Sprint acquisition.