Sounds like the MX does not have anything going on the would prevent WhatsApp from working. I would try reboot the MX, and then put your device in airplane mode (no cellular) and connect it to the wireless and try again. Also reboot the wireless router if you have one in the mix.
I also did that, rebooted everything but still no success.
One thing i decided to do is on my phone App settings (whatsapp) i turned off the possibility of switching to Mobile Data. Warning msgs stopped. But that is really strange, because seems now it is working... so maybe something related to the phone.
Will try to post something on Samsung Mobile communitites.
This sounds like DNS and similar to my internet experience up until recently. I had been using a 4G Huawei modem into my Meraki, and i had problems. Lots of them. All DNS related. Once i moved to a VDSL connection, the issue was gone.
They manifested themselves in almost the exact same way.
Well, all my "trusted" devices in my network are whitelisted. And no FW/Sec, IPS/IDS/AMP, neither L7 rules applied.
And problem still happens.
I am getting the following msg on my mobile (btw it is a samsung S10 - SM-G973F model) : "WhatsApp is using mobile data WhatsApp is using mobile data because the current Wi-Fi network is blocking it or there's a connection problem"
Any add suggestion for investigation is appreciated.