Welcome to the forum. I am not a moderator or employee. Just a user who helps for fun.
- Both wired and wireless connections? When it happens, does the light on your gateway change? If you have an older gateway with multiple lights, which lights are on?
- Does it only happen to android and iOS device? If so, those devices use random MAC address to avoid being tracks across networks via MAC. However, this privacy feature breaks the gateway's ability to know which device is which because the MAC address can change, and it changes to a randomized MAC which doesn't even tell the manufacture. What devices and what version OS for instruction.
- What does speedtest.net or fast.com say? With speedtest.net, there should be a way to share the results or just copy/paste the result ID they give you along the top after the test. Make sure to test on a wired/ethernet connection. Are you saying Cox's own speed test said 20Mbps? If so, did it show both Cox > gateway vs gateway > device speed? Last, check your link speed. Windows key + R > NCPA.CPL > Right click which ever adapter you are using and left click on status > speed. If it isn't gigabit, something is wrong with the ethernet or the port on the PC/gateway.
- Only thing I have heard that fix that is to try swapping the gateway out at a Cox store. The gateway is known to put out a lot of heat, so the new ones have fans. Since the fan is running a lot, it wears out soon I think, causing the noise. Since the gateways are furbished, you are probably inheriting a previous customers problem. Seems they don't take the noisy boxes out of circulation during refurbishment. Maybe because it saves them money. Not sure.
Based on your answers, I can help you see your signal levels from the gateway but won't go into that until I think it is required. What model gateway do you have? See here for types. Any of them should work for 100Mbps but I want to see if you have a DOCSIS 3.1 model because that has different signal requirements.
Also, have you considered buying your own modem and router instead of paying Cox for the gateway? It wouldn't help issue 1 and 3 but could help with issue 2 and 4. On top of that, retail routers have better hardware, more features, is a smaller target to hackers, easier to upgrade, more choices for both routers and different firmware, etc. Really the only thing you get using Cox's gateway is their support (which isn't that great IMO) and the ability to tie into their ecosystem for things like wireless TV boxes, homelife, and the Panoramic app. Main issue I use a retail router is so I can control when I upgrade the firmware and to what. Not long are some updates bugged, but I can download them directly from the manufacture instead of having to go through Cox's firmware testing group, which can both delay an update or outright deny it if it causes problems with Cox's network. So an important security update may not be sent out because it is included with an update that causes problem. Last, you can use 3rd party firmware like dd-WRT or Tomato.