Apps not loading on Roku Premeire

The 3920 is still receiving updates.  If a "System restart" found under Settings->System doesn't fix it, a factory reset is about all that's left.  You can try connecting to your phone's hotspot briefly if possible to see if that resolves it.  If it does, then we'd have to start exploring your network setup.

The 3920 is still supported (I have one). But it has some serious limitations compared to literally all current models. The biggest issue is the 3920 is single band WiFi, meaning there are speed limits to it's connection. Mine is within four feet of an access point on my network, and the best it ever reports is 22 Mbps, usually only reporting in single digits. My Ultra downstairs is within three feet of an access point, but because it support the 5 GHz band it reports over 200 Mbps (usually about 220). That is about the hardware limit of the Roku hardware, so don't expect better speed than that. 

The 3920 was released in Sep 2018, so now almost 6 years old. The Roku hardware has significantly improved over that timeframe, and frankly the 3920 is outclassed and can't keep up with some of today's apps/channels. I intend to remove my 3920 soon, and will probably move my Express 4K+ in its place. 

But all that said, I can still use Disney+, Roku Channel, Pluto, YouTube, and other channels without a problem. I don't use the others listed, but it would make me suspect there's a hardware issue with your device. A factory reset is a pain to do (since you have to set everything back up again, at least the channels will reinstall themselves) but if there's a glitch in the OS that would clear it out.