My actual intent is to deny all IoT devices internet access. I prefer to access them via a vpn server on my local net when I am not home. Not all “smart” devices can handle this gracefully. Can I still use the app (I use android) if it can’t get outside my firewall? What if the app can get internet, is on the local 2.4Ghz LAN, but the device is only allowed local network?
Thanx ahead of time for the info
The “I” in iot stands for internet so internet access is presumed. That said, most of the iot devices will have some functionality available when not connected. From my own experience with an extended internet outage, my cameras stayed active and I could live view them on my iPad which was also connected to my WiFi. However most alerts no longer worked as event videos could not be sent to the cloud. My contact sensors worked but again related event processing did not. YMMV.
Something to consider… I was playing with my router making configuration changes requiring lots of restarts and reboots the past week and it played wierd hell with my 12 device system (cams bridge/sensors). Devices went off line, stopped communicateing or needed battery pull to recover. When wifi is stable so is my system guess they get confused.
although I wasn’t adjusting my router settings…
I did have to power down my router a few times this past week, and there were a couple of power failures in my town, (we had a couple of tornados hit… First time in over 40 years.)
and what I’ve discovered is, of all my Internet connected devices in my home …the Wyze products do not like being disconnected… some will take hours or even days to come back online…
and others, a wyze bulb, and one of my v2 cams never have… (the bulbs have been the most unreliable… randomly and routinely disconnecting, (“offline”), for hours or even days at a time… i.e. My basement entry light has been “offline” for 3 days now… a dozen or so “powercycles” have not prompted it to reconnect. I’ve left it on for the last 3 days, in hopes that it will finally reconnect …glad it’s an LED, and only draws about 10 watts of power, but still annoying and a bit wasteful, but the only way to shut it off is to turn the electricity off to it and then it will never reconnect) 
all the other devices in my home, like Echo Dots, Spots, and Show devices, Fire HD tablets, Firesticks, Google minis, and chromecast’s, and a dozen or so smart sockets etc. …do not have this issue. 
I don’t know what to call them, but the type of outages we get tend to be either very short duration, I.e. flickers. Or full on outages lasting for hours. Of the two the “flickers” are the toughest to deal with. Usually my Hue Hub gets trashed and won’t work until I manually unplug it. Alexa is usually convinced someone stole our internet, and my Ring security system says we lost power, we recovered power, we lost power, we recovered power about 20 times. The Wyze cameras and contact switches either don’t even blink or become possessed and report either offline, not connected, or other various errors. Each time, luckily infrequently, is unique.