Phone not connecting to cell towers?

I just swapped the motherboard on an iphone 8, was wondering if anyone would know why the phone is reading no service when with a caution symbol beside the icon. The phone instructed me to update to ios 13 as some dual band feature was not working so I connected it to my computer and updated it. When the phone turned on it initially connected to the local network and even received a text message. However after 5 minutes it again could not connect to the network. Does anyone know if this is a hardware or software issue, maybe a loose connector inside the phone?