Why my iPhone stuck at searching, but work when sim slot is pressed?

The answer is quite simple: near bottom side of the Sim card reader you find a big IC called baseband that with bumps and flexing gets slightly disconnected from the logic board and oxide builds in between. Such IC takes care of all radio related tasks in an iPhone. When you put pressure you temporarily re-establish lost contacts. To fix your problem one should remove the IC, reinforce and/or rebuild oxidized rails and connections and resolder it correctly to the board. It’s a relatively common repair on iPhones 6.