you can take a train to shanghai, then take a train to hongkong from shanghai,

I don't think it is a good idea to take a bus from Yiwu to Hong Kong, too far. Well, if possible, you could take a train or a bus from Yiwu to Shanghai firstly and then take a train to Hong Kong. The train will take you about 19 hours and CNY400~600. The details could be found on the Shanghai Train Schedule(https://www.travelchinaguide.com/china-trains/shanghai-schedule.htm) .

On the other hand, if you will not go to Hong Kong in travel peak time, flying to Shenzhen via Shanghai or Hangzhou and then enter into Hong Kong on foot will be a reasonable method.