Solucionado, ya las recibo sin problemas. No sé que es lo que lo habrá solucionado, pongo aquí todo lo que he hecho por si a alguien le sirve.
1. Phone manager -> Power saving management->App protection->Disable auto cleaning for WhatsApp
2. Settings -> Permission -> You have allowed x apps to auto launch->enable WhatsApp
3. Settings -> Notification management -> WhatsApp->enable all
4. Phone manager>Power saving management>App battery usage alert>Never alert > whatsapp should be never alert
5. Make Sure the Apps are locked in Recent Apps. Lock means the app won’t be killed by system.
6. Third-party power management apps such as DU Battery Saver, Greenify, Battery Extender, CM Security, Battery Saver, Go Battery, Doze etc. restrict background data and thus result in delayed notification and errors
7. If still cannot receive the push message, please check whether Battery assistant is enabled. Battery assistant will restrict backgroud data and only suggest to enable when battery is low.
8. Dial *#*#46360000#*#* -> Uncheck “Enable background resource control”