As standard, you don't! Guest WiFi is primarily intended for guests who require WiFi, but access to other network infrastructure is locked down. Now implementations vary, but generally, you wouldn't be able to access printers or servers, but a few will allow access to streaming media servers.
If you have devices that require access to other pieces of local network infrastructure they will normally need to be on the main WiFi network.
I don't know if this is whats causing the issue, but I've recently seen articles talking about keeping WiFi-connected Smart devices on their own WiFi network, but in many cases managing this properly requires specific hardware and a little more management than the average user is used to!
*Paranoia Alert* Most smart devices do not have the processing power to crack the WiFi protection of other devices on your network!
