I have had my XS Max since yesterday around noon and can say that so far I haven't experienced any issues with WiFi or LTE connectivity. I have what appears to be the same or better LTE performance as compared to my X. Also, I bought mine from Apple and put my Sprint info in when I ordered it. All I did when I opened the box yesterday was remove the SIM from my X, turn it off, and then power on the XS Max. Activated during setup without any issues. Been loving it so far!
My Series 4 also just got here today. I had a Series 3 that I got back in June of this year on IB, and upgraded to the Series 4. Not sure how I was eligible, but when I was ordering the device I didn't see anything about getting a return kit with my new device to send the old one back. And when I go to Sprint's website I don't see anything about my Series 3 on it anymore now that I've activated my Series 4. I'm unsure when/if I can return the old device.