Bad battery replacement; Failed calibration?

Hi. Bought an iFixit battery replacement kit for an aging 6S. Performed the replacement without too much worry, however I felt like the battery connector to the main board was very loose, just floating there and only held down by the covering bracket (compared to the display connectors which all had a positive attachment).

Immediately after the install I charged it first via a faulty/intermittent charging cable, before rapidly transferring it to a better charger. Despite this, the phone never recorded a charge above 2% (after 18 hours charging), and died after a few minutes once unplugged. It seems to run on a boot loop even when plugged in, sticking to 1% or 2% charge. The fact that it doesn’t die immeditely after being unplugged suggests the new ifixit battery is indeed connected and working.

To me it sounds like my attempts at battery calibration have failed. Is there a way to redo this? I’ve tried disconnecting and reconnecting the new battery again, but it doesn’t seem to have done anything. Another possible theory is that because I possessed the replacement kit for about 4 months before performing the swap, somehow the replacement has degrated over time by not being charged. I don’t think this is likely.