Hi Patrik,
The first thing we need to figure out is if the problem is with your new battery or something else, possibly related to the installation. Assuming your old battery was functional if short-lived (and you still have it), the easiest way is to plug the original back in temporarily (it looks like you can do that without having to actually remove the new battery) and see if the old battery works correctly. If so, you've got a good case that you just got a defective battery and should look to the vendor for a replacement.
Otherwise, the next thing to look at is the connector itself. Check both sides of the connector with a magnifying glass, looking for any bent or broken pins. Sometimes bent pins can be straightened with fine-tip tweezers, but broken or missing pins are, of course, more problematic. If it all looks good, clean both connectors with some 90% or higher concentration isopropyl (rubbing) alcohol, reconnect the original battery and retest it.
Huawei P20 Pro Battery Replacement - iFixit Repair Guide
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