I had a similar problem.
You can't prevent it from going into sleep mode.
but...
According to recommendations I found online,
I made several changes to the sensor.
Goto Internet & Security >> Advanced - Developer Settings
Check the box of "Enable CoIoT"
Set the "CoIoT peer:" to <HA-IP:5683> (Exp. 192.168.1.107:5683)
Goto Sensor Settings >> Temperature threshold
Set "Temperature threshold" to 1
Sensor Settings >> Humidity threshold
Set "Humidity threshold" to 2
The above settings causes the sensor to update the HA more often effectively.
For me I have the temperature threshold at 0.5 and it's not enough. But I'll try with humidity to the lowest setting too and see if that helps. It's the curse when the environment monitored is too stable.
I think that a firmware where you can set it to wake up like once every other hour just to say it's alive would help.
The other alternative is to breathe heavily on it if it goes to sleep and becomes unavailable in Home Assistant.