Why is my iPhone searching for service/no service?

Hello, this is the first time posting here and I would like some help if possible.

I recently purchased an iPhone 6 Plus with a shattered screen (it worked fine had no SIM card though) from a friend. When I put my SIM card from my iPhone 5c into the new phone (didnt go to my verizon store yet) the 6 plus would have an active cellular reception. Heres the problem, after a while, the phone will lose service and begin "Searching for Service" or just say "No service" and it will randomly swap between working fine and having no service.

also, I use Verizon, and the phone is from the verizon network, but for some reason, right before it lost service most recently, instead of saying "Verizon" in the top left it said "Extended" which I thought was weird. I took it apart to replace the screen and I noticed what looked like the brown syrup from dried soda, so i assume soda has been spilled on or near this phone before :(

Any help will be appreciated!