I had the most issues when reassembling with this step. The plastic tool is a fail in my experience since not enough pressure could be applied using it. I just ended up using my finger and that worked 10Xs better. My advice, watch a few YouTube videos of people reconnecting the connectors and you'll be fine.

If you still have white lines after you've tried reconnecting the connectors and power cycling. Just do a hard reset a few more times, let the phone stay on and wait a while. As long as it's not damaged, the lines will fade over the next hour or so. Your phone will be as good as new.