Complex sentence: Luckily, British communications ships filled the gap until the SCR-299's could be landed, and once onshore, they provided the chief means of long-distance signals until permanent Army Command and Administrative Network stations could be installed.
Split into simpler sentences: Luckily, British communications ships filled the gap until the SCR-299's could be landed. Once onshore, the sets provided the chief means of long-distance signals until permanent Army Command and Administrative Network stations could be installed.