I'd handle it with one clear conversation and a small change to how you respond. In your next 1:1 say, 'Because we are across time zones, I am offline from 6 pm to 8 am my time. Send requests anytime and I will pick them up in my working hours. If something truly cannot wait, please call and put URGENT at the start of the text.' Then ask, 'Does that work for you?' so you get explicit agreement. When a night or weekend text comes in, ignore it and reply the next business morning with, 'Got this this morning and will handle it today,' which reinforces the boundary without scolding. If the pattern continues, forward one example to your boss and say, 'Just a heads up that I did not see this until morning because of my offline hours. Flag urgent items per our plan and I will jump on them.'
Back it up with phone settings so you are not tempted to peek. On iPhone, go to Settings > Focus > Do Not Disturb, add a schedule for your off hours, allow calls from Favorites only, and turn on Share Focus Status so your boss sees that notifications are silenced in iMessage.
In Messages, open the thread with your boss, tap the name or header, and toggle Hide Alerts so texts only appear when you unlock. On Android, go to Settings > Notifications > Do Not Disturb or Digital Wellbeing > Bedtime mode, set a schedule, and under Exceptions allow calls from starred contacts. In Google Messages open the thread, tap the three dots > Details > Notifications and set it to Silent. For work apps, set quiet hours too, like Slack Do Not Disturb or Teams Quiet Time, and use Schedule Send in Outlook or Gmail so you do not train people to expect off hour replies. This worked for me in an always on shop because I repeated it calmly and never got snippy. Put your working hours in your chat status and email signature, mirror the courtesy by scheduling your own after hours messages, and your boss will usually adapt within a week.
