Timeout when connecting to a local webserver through the internet, but only on WiFi

I've recently moved, so I have a new ISP and I've also switched to new network hardware. I've been pulling my hair out trying to understand why I keep getting 100% timeouts when connecting to a locally hosted website. To make it more complicated, it only happens when I’m on WiFi.

Hardware setup is:

ISP router/modem -> Ubiquity Cloud Gateway -> U7 Pro AP -> Laptop
                                           -> Webserver

The issue is opening https://foo.bar.baz:58443 when on WiFi. This domain points to my home (not really bar.baz, but you get the idea). There's is port forwarding rule to get to the local server. With tcpdump, I see the request coming in on that webserver, a SSL handshake is completed and then a bunch of TCP retransmissions.

Some observations:

• If the machine with the browser is connected to a cable and not WiFi, everything is fine, no timeouts.
• Opening https://192.168.1.123:58443 (webserver address) is fine (WiFi or wired).
• Opening https://10.0.1.123:58443 (gateway address) is fine (WiFi or wired).

I thought it would be MTU related, but haven’t had any luck with changing it to a lower size. I’m not positive I’ve done this correctly, though, so it may still be MTU related.

I know there are people here that know way more than I do about networking, so I hope somebody can point me in the right direction.