No Stupid Questions

Oh, that's a good thought.

Hmm. Well, I don't think that it's only using some new WiFi version that your older device can't speak, or I don't think that they'd be able to see it.

Well, three guesses.

First, the wireless password (or case?) isn't what you think it is, especially if you're hiding the password as you input it. Like, maybe on the iPhone you just got lucky and got the case right. Might try re-entering the password on the iOS device.

Second, if you set the new router to have have the same SSID (network name) as the old...maybe some devices remember information about the WiFi connection, and so when you're trying to connect to the "new network", it's trying to connect to the old one. Linux's NetworkManager retains information like this, as does Android. If that's the case, there's probably some option to "forget" or "delete" the network entry on the connecting device. Do that, rescan and hopefully you're talking to the new network. I don't know if it's possible for two networks to be similar enough that the device would confuse them while retaining some kind of information about the old one, but that might do it.

Third, it could be that the router uses some sort of quirky authentication mechanism. I remember reading about some users at a few institutions running into that (maybe...they were doing something with PEAP-MSCHAPv2 with some weird option or something that prevented authentication on a bunch of Linux devices...don't recall). But I don't think that I've read about that coming up with PSK, which most home users are probably using in 2025 (and I'd think that you are, if you're only being asked for a password, not a username). You might look in the wireless network settings of your iPhone that can connect and see what it says is being used as an authentication mechanism. If you have a Linux box that you're trying to connect with, you can dump pretty extensive information like this:

$ nmcli connection show

That'll give the name of all known "connections", remembered WiFi networks. This is typically, but not always, the same as the SSID.

$ nmcli connection show <name-of-your-network-connection>

That'll show extensive information on the connection. I think that all the authentication information is under 802-11-wireless-security. Might give some clues. You could post it, though I'm not really familiar enough with it to advise doing so. On my system, it looks like all the sensitive information is hidden with "", like key data, though I haven't played with it enough to say that just posting all the lines starting with 802-11-wireless-security wouldn't provide any sensitive information.

That's kinda all that comes to my mind, at any rate.

EDIT: It's possible, on most wireless access points that I've seen, to blacklist (and maybe whitelist?) the MAC of a particular device. That'd maybe produce that "can't authenticate" behavior, though I can't imagine how you'd wind up with your phone having a different status from anything else.

I guess it'd be theoretically possible for a WAP to limit how many devices are connected. I've never seen that in real life, but might try disconnecting the iPhone before connecting the other device.

I'd think that you should not need to do this, but one last-ditch fix, if nothing else works

you can stick your own, dedicated wireless access point on the network, connect it to the broadband network via the wired network, then connect to that wirelessly from your devices. Then you're not constrained to use your ISP's wireless access point.