Recommend phone for timely WhatsApp delivery

Hi,

I am trying to configure Skedit to send a Whatsapp at midnight each day. I have tried on a few phones and it sometimes fails or is send a few minutes late. 

Due to this COVID-19, there are limited booking slots for the swimming pool where I swim. Booking is open three days in advance so the only way to secure a slot is to stay up until 12:00 am. All slots are gone by 12:01.

I am trying to use SKEDit for this purpose. I have tried a few old phones and they often fail or are sent late. I am using the phone with its own SIM for this purpose only. 

I am having a lot of problems. I tried a Huawei G8, Xiaomi Mi A1 and HTC one X but I cannot keep the service running so that messages do not periodically fail. I can't find the autostart setting on any of these phones. 

Any advice would be greatly appreciated. 

Kind regards,

Luke