When I went to switch my Ubuntu Raspberry Pi 4 from wired ethernet to WiFi, it didn’t work. I was able to see the networks on my router and sometimes even to connect to them, but it wasn’t stable. I bought a second Pi 4 thinking it might be a hardware problem with this unit, but it was exactly the same. I read about some problems with interference from HDMI but I tried three different monitors and didn’t see any change, but I suppose that could still be the ultimate problem.
I originally thought it was a power problem, with my USB HDD perhaps drawing too much power and destabilizing other parts of the system. I put the HDD on my powered USB switch but that didn’t help. I eventually bought a USB WiFi dongle (TP-Link PC N150) which was better but still not acceptable.
I wondered if the power problem might be with the USB WiFi dongle so I moved it to the USB switch and suddenly things seemed better. Then later it got worse. I eventually realized it depended on where I moved the USB WiFi. Closer to the Pi 4 was bad (0.2 MBits/sec) while further away was as much as 20 Mbits/sec.
I wondered it if the power really had anything to do with it. I got a USB extension cord and put the USB WiFi dongle on the end. I can move it closer and watch the WiFi degrade and move it away and watch it improve.
One last bit. I have a wireless keyboard and mouse with a similar USB dongle. Thought it might be interfering with both the on board and the USB WiFi, but I went back to a wired keyboard and mouse and there was no change. I do wonder if the on board WiFi might be causing problems with the USB WiFi. It does seem like Ubuntu won’t let you selectively turn off WiFi. It only does “Airplane Mode” which turns both off.
Either way, there is some serious flakiness with the Pi 4 WiFi. For now I’ll just use my USB extension cord and USB WiFi Dongle.