You can take a bus from Ranong to Krabi via Chumphon and Chumphon in around 9 h 5 min.
How to get from Ranong to Krabi
From Ranong Mini Bus to Chumphon, walk to Chumphon, then Minivan to Krabi Bus. 9 h 5 min ฿853. From Ranong drive to Krabi. 4 h 41 min ฿1049. From Ranong taxi to Lang Suan, train to Ban Song, then taxi to Krabi. 5 h 50 min ฿6110.