Secure Client for DigiXbee

No. The S6B (wifi) does not support secure clients.

The ESP32 does.  It’s also a currently supported product and very widely available.  You can get very cheap ESP32 breakout boards in a variety of shapes and sizes.  There are several flavors depending on how much power you need, but they can all do a basic secure post request.  You could even use the older, even cheaper ESP8266.

If you’re using something else as the “brain” and only want to use the ESP32 (or ESP8266) as a blind modem, flash it with AT firmware and use TinyGSM.  Check out the Web Client example for a hand-made post request.  This is probably similar to what you were doing with the S6B.  If all you need is sever certificate verification, that example should be all you need.  If you need client certificate or mutual verification, I wrote up an example here of uploading new certificates to the module.  You only need to upload the certificates once.

You can get a lovely ESP32 in Bee format with AT firmware pre-flashed here:  https://www.envirodiy.org/product/envirodiy-esp32-bee-wifi-bluetooth/

If you don’t want to use a second board as the brain, there are zillions of examples on the web of people using ESP32’s to do all kinds of cool things.  They’re very heavily used in home automation and are quite powerful.  Buy any breakout format you want.