
If you have many apps installed, it's quite tedious to set all apps to treat their notifications as priority (perhaps a script might help.). You sacrifice the intended usage of Priority only mode, only leave Total silence as the only option (however, if you never intend to use this feature, then you're fortunate since you can utilize it). However, there are some disadvantages that you might consider first before using this workaround: Leave this app as non-priority, but don't forget for the rest of system apps). You need to tick Show system from overflow menu. The idea is to treat all notifications as priority (from Settings - Sound & notification - App notifications - Treat as priority) except for captive portal notification, which is under Android System app (by default, system apps are not listed. However, only priority notifications will be alerted (sound, vibration and LED light). Priority only mode will still show all notifications. To put it simply, you must use Priority only (from Quick Settings - Do not disturb) since it has per-app setting for notification alert that can overwrite system's default setting.
There is a workaround involving Do not disturb's Priority only mode.