Dexcom G6 and "automatic IOS updates"

Because our iPhones were set to "AUTO" updates, my husbands IPHONE 10XS is no longer compatible with his Dexcom app (which gives blood sugar readings every 5 minutes - and he is Type 1 and kidney transplant patient). This is key to his health and apparently you can not "DOWNGRADE" back to 13.6 or lower. We have taken out the auto update going forward but now can't trust phone. Anyone have this problem and how did you handle? Also found out that his Dexcom receiver is out of warranty. HELP!