Yan Fei, though not particularly worried about the troubles the US forces outside might cause him, didn't mean he planned to just sit and wait for doom. The main reason he hadn't taken any action before was that he wanted to delay time and wait for nightfall.
Previous oversights made it clear to Yan Fei that the United States' satellite technology was highly developed, especially over the United States mainland, which is almost entirely under satellite surveillance. Otherwise, the United States wouldn't have discovered Yan Fei's whereabouts so quickly. In clear weather conditions, as long as Yan Fei appeared outside the cave, there was a great chance that satellites in outer space would detect and lock on to him. So, no matter where he hid, as long as it was daytime, he would eventually be discovered by US forces. That's why Yan Fei decided to wait for nightfall.