Seems I cannot reply to your last private message. We have 3 scenarios indeed:
No NAT used, no VLAN ID. WhatsApp seems to work fine on it.
No NAT used, VLAN ID in use. WhatsApp not working properly.
NAT used for Guest SSID. WhatsApp not working properly.
Waiting to hear back from engineer assigned...even though according to his reply NAT seems to be well known for causing issues with WhatsApp - not sure if you had the chance to read the case notes about this.