The way your iPhone system is set up might have an impact on how some apps function. You may, for example, set your device to restrict particular applications from auto-updating and to allow or deny an application system access. However, it's conceivable that how the Facebook app functions are related to how relevant settings on your device are set up. If your Facebook app started acting strangely or crashed after you changed certain settings on your iPhone, this is most likely the culprit. You can restore these settings to their prior state to rule out this possibility. If you can't figure out which settings or choices to alter, you may just reset all of your iPhone's settings. You can enable relevant features or options separately and based on your preferences after resetting all settings.

Go to the Settings-> General-> Reset option on your iPhone 7 Plus to reset all settings. Then touch the option to Reset all settings. To proceed, enter your device passcode if requested, and then accept the reset of system settings.

Any data on your iPhone's internal memory will be unaffected. Restart your iPhone after the reset to see the changes. After that, the default system settings will be applied. All relevant functions, such as Wi-Fi and Bluetooth, should be enabled.