How can I sign out of another device when that device is not active but is using Wi-Fi to see my information

Basically my fiancé is a control freak back when I had no phone he loaned me his old iPhone 6 I logged in with all my credentials Apple ID and so on. Then I got a new I phone and logged in with the same apple if and iCloud account. I started noticing weird little things happening as if some one else was controlling my phone. I would even here a phone in the basement vibrating every time my phone would go off. I said something to him but he denied it called me crazy. And so on. So then it hit me I never logged out of anything on the iPhone 6 he loaned me once I got my new phone I just handed him the phone back. Now although the iPhone 6 is not under any provider like spectrum, Verizon and so on bc it’s just a spare phone so no need to pay another cell bill for a phone when we both have our upgraded iPhones . However the iPhone 6 is connected to our in home internet using our Wi-Fi-network. So bc the phone is using that it’s able to turn on and see my i messages messenger my fb what I do on google even my pic and Calender and notes. But when I call my phone provider and ask if I’m logged into any other devices they say no . But that’s bc they can’t see that I’m still logged into the old iPhone 6 bc technically that phone is just a phone using Wi-Fi not an actual phone carrier. Does that make sense so I’m trying to figure out how can I logg out of that device using my new iphone. Bc basically it’s like my fiancé has a side phone that is mirroring everything I do as if he’s Literally looking at my phone as if I’m sharing my screen