This sounds difficult. Most of us with ADHD face the reality of lost items. I used to Think of it part of my hard wiring. I labeled myself as one who can always be counted on to lose things and the higher the value the item had the more likely I was to lose it.
After receiving my diagnosis I started looking for the frustration and lost time and energy. The book ” Help ADD Stole my Car Keys” was helpful.
One of the authors I have read introduced the concept of launch pad. Simple solution, difficult to start but highly effective. Start small. Make a few rules. Cell phone ringer always on. Phone on bedside charger every night. Pebblefind alarm on key fob , keys on a hook by the door. Wear something with a pocket that can hold the phone. (Women’s clothes are notorious for this lackn so I have started wearing dockers. Or my husband’s gym shorts over my leggings). Papers that I need to take somewhere are always placed at the unused end of dining room table on a tray so if company is coming I can quickly move the mess out of sight.
Repeated and constant repetition will set patterns that don’t require working memory.