WiFi connection rapidly drops and restarts when I try to use it.

I just got myself a refurbished iPhone XR, and it's working well except for this. More or less at random, the wifi will suddenly and rapidly disconnect and reconnect over and over. And I do mean rapidly, like cycling back and forth literally every second. It only does this when I am actually trying to use the network. When it's idle or I'm doing something offline, the connection appears intact. But when I try to go online one way or another, it will sooner or later start to fail. I can watch the symbol disappear and reappear in the status bar, or the actual network list flickering in Settings.

This was happening right out of the box: while I was still in the initial setup, it would do this while I was trying to download the latest firmware. I had to restart the setup a few times because it seemed to keep going wrong. It was to the point where, having left the update downloading overnight, by the morning it was only like halfway done. Only then did I pin down that the network connection was what was having trouble. And getting it set up didn't fix it, either. While I was able to update and complete the setup through iTunes, it still has been happening frequently during use.

Oddly enough, it seems to have slowed down as I've been using it. It hasn't stopped, and it's still happening enough to be a noticeable problem, but during the first few hours it would cycle almost every minute when I tried to go online, and would stay that way for a solid 15 seconds. Compare that to now, where I can get a fair bit of time online before it happens again. I'm not sure what to make of that, but it's almost more annoying than had it just stayed the same; the inconsistency is very strange.

This only happens on wifi; cellular has no problems staying connected. And it happens on all networks, not just my home one. Likewise, none of my other devices have a problem connecting, even right next to each other. So it definitely seems to be a problem with the phone itself.

I have tried the following:

• Rebooting the phone
• Rebooting the router
• Connecting to a different network
• Turning WiFi on and off
• Turning Airplane Mode on and off
• Updating to the latest firmware.
• The "Reset Network Settings" option.
• Setting the phone up as a new device.
• Setting it up with a backup from my old phone

And none of those have worked. The problem still occurred at about the same rate each time, barring the gradual and inexplicable slowdown.

I have a window where I can get it replaced, so I might just do that, but I did want to ask around before doing so. Does this seem like a hardware problem? Some sites I have read say that it could be the antenna itself that's damaged if none of the other tricks have worked. And if that is the case, is it worth trying to repair that myself instead of just exchanging it? Or is there a simple software solution or something that I'm just missing?

(Apologies if this is too rambly; I tried to provide as much detail as I could, although I can provide more if I forgot or missed something. I appreciate your help. Thanks!)