Macbook Pro 13" 2015 A1502 new Battery not detected, old battery just suddenly stopped working over night.

I have a old MBP2015-13". It had an issue with the trackpad and keyboard not working at all, eventually I found info on the internet after months of searching and finally decided to replace the ribbon cable, and found a pin on the motherboard connector of this cable bent, bent in place, and the trackpad and keyboard worked. Wow great, I was super excited as I thought I could finally start using the laptop like normal.

However, at some point just before this, the laptop stopped working completely, I eventually removed the battery and somehow got it to power on. When this happens the fan spins at full speed. It doesn't stay on for long either. I reconnected and disconnected in multiple attempts to see if any of this is working or not. Managed to get the battery connected and powered on. In System info, it does not show the battery at all. Thus i thought battery is done for. So eventually bought a replacement battery. Managed to change the battery, but it is still the same. I did do a SMC reset, but this seems to make worse when trying to turn on the laptop, sometimes it simply refuses to come on, until I play around with power button/power cable connect-disconnect/reconnect-disconnecting the battery and various configurations.

I don't have the money to take in for professional assessment, also I'm from South Africa, and finding reliable and reasonable priced repairers is almost impossible. This is the reason I am trying to fix it myself. However I have hit a brick wall as to what this could be now. Charger works fine. I would really appreciate any support. I am able to some level of board testing via Multi-meter and de-soldering. I'm not a professional, but been working on and with computers as far back as 1995 as a hobby. Thank you in advance for any help.