Hi,
As per the setup you have mentioned, it seems the ISP Modem LAN port is directly connected to the SG200 switch where the Access Point is connected via which wireless clients are associated. First, please verify what is the Modem LAN interface IP configured. In case the same IP is configured on another device on the same IP Network then you will face device access issue using that duplicate IP.
I hope the Modem LAN is configured with DHCP which is providing the IP address automatically to the other devices connected on the same LAN.
For example, if the LAN IP of the Modem is 192.168.1.1 /24 and DHCP configured on that then the devices connected in that LAN will get the 192.168.1.x IP via DHCP. Until or unless you know that DHCP IP assigned on the switch or WAP, you will not be able to access the GUI of that device using that IP. In that case either you can configure a static DHCP binding on the Modem (if the option is available) so that the switch or Access Point will always get that particular IP or you can configure a static IP on the Switch and Access Point from the same LAN IP subnet. And you need not to disconnect the Modem in that case to access the Switch or Access Point.
