Apple Pay not working after frame replacement

You likely either damaged the NFC antenna, or installed it into the new frame incorrectly.

While iFixit does not have a guide for the NFC antenna (or other, more difficult internal components), you can see it in the top left corner on steps 13 and 14 of the teardown guide. iPhone XS and XS Max Teardown

In fact, at the end of the teardown it is still in the phone, never removed, meaning iFixit's own staff never removed it from the frame as part of the full teardown.

When doing a full frame replacement, this antenna is delicate. It is glued down to the frame and very easy to tear or damage during removal and reinstallation. It is one of the very last things you can take out of the phone, requiring you to take out most other components before you can access it. Additionally, all of the screws serve as grounds for not only the NFC but also other components that use the same screw holes.

The above is an image of the antenna. If you lost any of the screws that hold the antenna down, or if you layered the parts the wrong way, you could have simply not grounded the antenna correctly and it will not work. It could have easily torn (even just a little bit where you did not see). Or, finally, it could just not be plugged in correctly. Sometimes it is such a small part and so tucked into the corner of the phone that people do not even recognize they need to transfer it over.

Consult this image to find your antenna in your phone. Verify it is in good condition, and make sure all the screws are layered correctly (i.e. such as the NFC screw hole directly against the frame, then the screw hole for <other component(s)>, then the screw on top, with any standoff screws in the middle properly as needed.

Apple intentionally makes these things needlessly complicated, complex, and difficult to repair properly. They give an unnecessary amount of screws of different sizes and shapes and types and they design their parts easy to accidentally damage during user removal and the ground specifically to short and not work properly if it is not done precisely correctly according to Apple's specifications. Do not feel bad if you messed it up, the NFC antenna is notoriously one of the hardest ones to get right on a full frame transfer.