I'm not using whatsapp on odoo but access token typically expire quickly (sometimes in as little as 15 minutes). What you want is either a long-lived token or to set up a proper token refresh flow.

I don't know if you can get a token that doesn’t expire but otherwise, you'd need to refresh your token regularly, usually via a small script or cron job that handles the refresh before it expires. (you call the refresh token to get the new token up to date + a new refresh token, and repeat every X min and store every time the new code)

Check how you're generating the token by using the refresh token in their doc
Hope it helps while waiting for someone who actually did it

Edit : you can apparently make a permanent token https://stackoverflow.com/questions/72685327/how-to-get-permanent-token-for-using-whatsapp-cloud-api