All 4 wi-fi networks (2.4Ghz and 5Ghz on each router) have the same SSID and password. I think this means my devices should be roaming freely between them, and it roughly seems to be working, but I'm not sure how reliable it is - especially with the Nintendo Switch which is notoriously bad at wifi roaming. Either way, as far as I know neither of the routers is aware of the other's wifi, and I'm not sure if that's supposed to be the case.
The TP-Link firmware on the C7 has a "Smart Connect" feature enabled, which seems to mean that the 5Ghz and 2.4Ghz networks on it are somehow supposed to hand-off better..? but I'm really not sure.
Anyway, is there anything you would suggest I change about this setup? As I said, everything seems to be working and I'm getting good speeds on either router. But if there is a way to make the routers take a more active role in roaming my devices to the best network (especially the Nintendo Switch!), I'd like to do that. If needed, I can of course install OpenWrt on the C7.
It's been years since I needed to have a gateway with radios on and an AP with radios on working together, but as I recall, I had two physical "routers" in the mix as you do. The one connected to the cable modem was configured as a gateway and the other as an AP. There was a bit of setup required to differentiate them.
I don't think I ever saw wireless devices (phones/laptops) successfully switching between the two. They would just connect to one of them even through the signal got weak. As I recall, I ended up switching the radio on the gateway off and just keeping the AP's radio on and that gave more consistent results and decent coverage. Keep in mind, my experience was using hardware and firmware from 8+ years ago.
I did not run both 2.4 and 5.0 radios together but having the same SSID 4x seems wrong to me.
Why not run OpenWRT on both? I am too lazy to browse to the supported table of hardware. Perhaps your other device is not supported (thought it was)?
I can, but I'd rather not if there's no real benefit for it. At the end of the day, the stock firmware is just far far easier and more friendly to fiddle with. With OpenWrt (which I've only started using for the first time this week) I worry I'll mess something up and worsen performance or expose myself to attacks in some way. The "dumbed down" stock firmware has its merits.
Still, as I said, if there's an actual benefit or preferable configuration I can easily do that. The page for my router says that installing OpenWrt is as easy as uploading it to the router's firmware upgrade page, no need to even set up TFTP like I did for the C2. It's configuring it that I worry about.
I have 2 floors, each router does a pretty good job covering its own floor. In my experience (and also according to online information), most modern smartphones do happily roam between networks if one signal is better, especially with 2.4GHz vs 5.0GHz, since generally 5GHz has greater speeds but 2.4GHz has longer range. I might be wrong though.
From all the information I've managed to find, in all cases it's the device's responsibility to decide to roam when the device wants to. There's only a handful of "little pushes" the APs can do to steer it, but they really are little and they all have some cost. One page I've found says:
[[ STA (station) will be used to refer to wireless client devices. ]]
The STA is the boss when it comes to roaming, making almost all decisions about when and where to roam. But the AP can influence the roaming process through the methods below. In all cases, however, the STA makes the final decision on where to roam.
Delaying responses to STA authentication requests. This is typically used to band-steer STAs by delaying the authentication probe response on the band the AP doesn't want the STA to connect to. Since the STA sees the AP on the preferred band respond first, it usually connects there.
Refusing to authenticate a STA. This is used to limit AP load, i.e. number of associated STAs.
De-authenticating, i.e. forcing a STA to disconnect. This is usually done as a last resort, for obvious reasons.
Providing an optimized list of APs to roam to, aka Neighbor Reports (802.11k)
Providing information about the traffic load on other APs (802.11v)
Speeding up the roaming process by providing fast authentication (802.11r)
Influence is the important word here. To paraphrase an old saw, you can lead a STA to an AP, but you can't make it connect. You can also disconnect a STA, but you can't (or shouldn't) make it stay disconnected. APs play hardball with witholding authentication or enforcing deauthentication at the risk of being blamed for being flaky. After all, the network was working just fine before you changed to that fancy new router, wasn't it?
Specifically with the Nintendo Switch, I think it just ignores all of these little pushes, determined to stay connected to whatever AP it's already connected to, signal strength be damned.
Edit: damn, wish I'd found that page sooner, it clarifies a lot
Edit: you know what? Thank you (genuinely) for trying to help, turns out I should just leave my network as-is. It's working, there's nothing to fix
Unfortunately we don't own a Nintendo Switch, would have made a simple test, and moved across our three floors, to verify whether it stays with one AP, or not.
Probably also giant security holes that are never patched by TP Link as they've moved on years ago to newer hardware
Your site is good and gives correct information about station roaming. One thing you can do is turn DOWN the output power of your APs so that they can't be heard on the opposite floor. Voila the device will have only one option on each floor.
No, you've got it all wrong! It's actually been updated just last month. The latest update fixed the wifi instability that the previous update introduced
Did you consult the wiki? A quick search found: 
