The card works independently of the camera model. It serves as normal SD memory and the Wi-Fi is one level behind it. The only disadvantages are that the memory is not very fast and the battery is constantly drained of power. On the plus side, the pictures end up elegantly on the mobile with the app. I am very satisfied.

The card itself sends a wifi signal that establishes the connection between the camera and the mobile device.

Since the EOS 1200D supports SDHC cards, the card/camera combination should work without any problems. For the receiving device, however, the supported operating systems should be taken into account: https://www.toshiba-memory.com/wp-content/uploads/2019/07/Change-notice-of-FlashAir-App-OS-Compatibility_EUMEA_190618.pdf

Google "toshiba flash air camera" zweiter Treffer: https://jp.toshiba-memory.com/ww/compati/flashair/pdf/FlashAirW-04_Operation_checked_device_list.pdf

I don't know much about Nikon, but the D810 is on the compatibility list for the W3 model (the model sold here is W4), so it should work. See https://www.toshiba.co.jp/p-media/wwsite/compati/pdf/flashair_w3.pdf I use the card in a Sony RX100 (1st generation) which is also on this W3 list, and it works flawlessly.