If the original battery when reinserted shows that it is charging and the tablet will boot from it and the replacement battery doesn’t show that it is even charging then it appears that the problem is with the replacement battery.
If you have a DMM (digital multimeter) use it to do a static comparison test between the two batteries. (static testing means no voltage is applied to the component under test)
With both batteries disconnected, use the DMM’s Ohmmeter function and test between the black wire and the yellow wire on the battery cable plug and then between the black wire and the white wire on the cable plug and check if the measurements for both the batteries are the same or very similar. If not then the battery management circuit in the replacement battery is most probably faulty. If one of the test measurements on the replacement battery indicates an open circuit then there may be a wiring problem between the battery and the battery cable plug
If both the batteries have the same or very similar test measurements, then change to the Voltmeter function in the DMM and measure the voltage across the red and black wires for both the batteries and check the voltage. The replacement battery should have some vestige of voltage in it. If no voltage is present it may be depleted beyond recovery or there is a wiring problem from the battery to the cable plug.
If you don’t have a DMM contact ifixit on firstname.lastname@example.org and tell them that the replacement battery doesn’t work and that the failing original still does when reinserted and get a replacement.