Hello what hardware to check in order when network issues with iPhone

I got an iPhone 5s, and network works randomly since I got it, goes from working to no service, or sometimes searching for hours, while being at the same location where there is a good network, as usual I have done reset network, reset all settings, restore it, press on/off + home button but the issue keep persistent. so I don't think it's an issue with the iOS, so what should I check in order in the hardware to make sure everything is working fine! I mean it could be a cable displaced, some oxidation, too much dust in the iPhone, problem with SIM card tray who knows?

So is there a specialist that can give me the right direction to operate to check the hardware with a step by step instructions?

I got to add it was a second hand phone, and the previous owner didn't seem to be really looking after it, as I had some issues with the headphones, sim reader and charger plug, I just had to clean them with alcohol 70% to solve the problems, cos it had a lot of dirt in them ( apart the sim reader).