Hi Hanna,
Have you downloaded the pdf service manual? You might find it useful. Shows all screws, etc.
https://usermanual.wiki/Lenovo/330s14ikb330s14ikbx330s15ikb330s15ikbxhmm201803.2053639346/help
The connector with no ribbon cable connected just the the left of your yellow circle '''''is''''' connected in the service manual. I would investigate this. It seems to come from under the battery.
The black and red wires you mention are for the RTC or cmos battery, which stores the BIOS or UEFI settings and the time, but this would not stop it powering on.
It is vitally important that all cables are correctly inserted. All the parts of the machine are interconnected and dependant on each other. The problem may be that some thing you did incorrectly the first time you have repeated each time. Read and study the service manual and review what you have done….oh and always shut down and disconnect the battery.
I still suspect your problem is just a cable especially as you get no power lights what so ever.
Let us know how it goes. Everything has a cause. Every problem has a solution and usually a simple one. The hard part is finding that simple solution.