Some apps do not detect an internet connection

I've got a bit of a bizarre problem with my Android phone: My internet connection is working and fully functional, since Chrome and most other apps have no problems. However, some apps do not detect an internet connection. They display 'No internet connection' errors or similar. Some apps do this far more frequently than others, and some can reconnect randomly. Here are some observations: - My banking app does this almost every time I want to check it. - Various messaging apps will receive messages hours late (sometimes). - This issue is temporarily resolved on reboot, and for some time all apps can connect to the internet. - Terminating a process and relaunching it does not let it reconnect to the internet. - Some apps can be using the internet just fine at the same time that others are not. - Some apps never do this, such as Chrome, Google, various games and others. As far as I can tell, it has existed since I got the phone a few months ago. I've tried disabling Samsung's battery manager in case it is messing with apps to save power. I'm currently running my phone in safe mode - but it's unclear if this has had any effect yet. I'm on WiFi, and I don't use mobile data. I own a VPN app but it's almost never enabled (except when traveling). The VPN app is one of the badly affected apps. Other devices on the same WiFi network do not show similar issues, and this problem occurs on any WiFi network. Any advice on how to fix or troubleshoot this issue would be appreciated.