We don't know what you would regard as viable.
I would still be using a 3GL flip-phone not a smartphone — but my USA service provider withdrew 3GL and forced an upgrade to 4GL VoLTE unsupported by the old flip-phone.
"standby time of 300 hours" https://www.cnet.com/reviews/kyocera-kona-review/
Maybe there's something here for you —
https://www.nokia.com/phones/en_us/feature-phones
Nokia 6300 4G supports WhatsApp.