Concidering that the battery was working for a while after the installation, there may be a defect with the battery.
There is also a good chance that the battery is not connected properly. Possibly the data line of the battery which would cause odd issues as you’re describing.
If you still have the old battery, it may still be able to charge a bit. You could use it to confirm that the new battery is at fault if it charges more then 4%. I would try to reconnect the new battery first.
If none of those seems to work, there is a chance of damaged motherboard components, or partial damage to the charging flex cable. These are less likely since the watch was working for at least 2 weeks after installation.
I hope this helps