What a guest network is for
A guest network gives visitors internet access without giving them access to your things. Someone on it can browse and stream, but can't reach your network storage, your printer or the laptop in the next room.
Routers do this by keeping guest devices away from your main network, and usually from each other as well. Often the guest network sits on its own subnet too.
Why transfers fail on it
Two devices on a guest network are normally stopped from exchanging any traffic directly, so discovery finds nothing and a transfer has nowhere to go. That isn't a bug. It's exactly what the setting exists to do.
The same applies when one device is on the guest network and the other is on the main one. They share a router, but they aren't on the same network in any way that matters.
How to tell which you're on
Guest networks usually give themselves away in the name, with something like -Guest on the end. If the naming isn't clear, compare IP addresses on the two devices. Different third numbers, like 192.168.1.x against 192.168.2.x, mean different subnets.
Some routers apply client isolation to the main network too. If devices on your primary network can't see each other, that setting is worth a look in the wireless configuration.
When to use one anyway
Guest networks are genuinely useful. Visitors are a good candidate, and so are smart home devices you'd rather keep away from everything else.
Just keep the devices you transfer files between together on the main network. There's no way to transfer across the isolation boundary without turning the isolation off, which would defeat the point of having it.