My iPhone says "Searching" all the time.

Before my phone was experiencing any cellular issues, it was working perfectly fine. Then one day I brought an new phone then held on to the one i already had. one day the phone i brought broke and this "broken" phone was my back up. I turn it on and for like an hour I had service until it said "Searching." I called apple and even walked in they told me that nothing is wrong with the phone, they ran all the tests on it and cannot find any issues. They suggested it was my sim, so they gave me a new one to take to tmobile to activate. I go to tmobile and activate the card then put it in and stil nothing.. Ive tried disassembling my iphone to the part where you get to the "L board." I replaced the antenna or wifi antenna and still no service. After that, I replaced the whole L board with another iphone six and still no service, but the other iphone has service with the part I am supposedly having issues with. I even risked my phone life by dropping it and still nothing, haha. Tried taking out the sim with the power off and still you can guess.. HELP?????