The PDIP version of the RTC RAM is long discontinued meaning all of the existing stock is new old stock with aging batteries. So, if you can't find a working chip and you do not want to dremel out the battery and replace it yourself, you can instead use the SOIC version. It has an identical pinout but needs an adapter to allow it to plug into the socket that you've installed.
Get any SOIC-28 to DIP-28 SMT Adapter (1.27 mm pitch, 600 mil body), such as Chip Quik's
PA0011C, and then get the SOIC version of the chip which has part number
M48T58Y-70MH1F. This is paired with a Snap-Hat battery (meaning the battery is changeable without replacing the whole chip) which has the part number
M4T32-BR12SH1.