I ran into the same problem of not being able to connect with Google IoT, Azure, and AWS. I did finally find the root cause and a solution about 5 months ago. The root cause is that MKR NB 1500 ships with outdated firmware for the Ublox SARA modem. The procedure to fix this may seem a bit complicated, but trust me it is not too bad. Read through the thread below carefully several times and it takes you through the process step by step. There are several things you need to download but all the instructions and locations to get that stuff are in the thread. I will tell you upfront so that it's not a surprise when you get to the thread, you have to solder a USB cable directly to the modem pins on the MKR NB 1500. For me, that was the scariest part, but it turned out to not be too bad.
Here's the good news- once I did that I was able to connect to AWS in a snap. Everything I had been banging my head on the wall about was fixed. I have not tried Google or Azure since then, but I'd bet this will solve the issues for those platforms too.
Good luck!
https://forum.arduino.cc/t/firmware-upgrade-for-ublox-sara-r410m-02b-on-the-mkr-nb-1500-2/699292