More details would be needed on your Hotspot configuration.
Also, if you are trying to access the device on wireless connectivity, it is normal to not be able to do so as Hotspot is designed to prevent connected users to access your LAN, yet provide connection to the internet. Are you able to login using wired connectivity?
There might be a chance that the hotspot network subnet you have configured is the same as your LAN network. Due to there being an IP conflict you do lose access to the device.
The solution would be resetting the device to factory defaults and reconfiguring Hotspot again.
Device factory reset options are provided in the following link: https://wiki.teltonika-networks.com/view/RUT240_Device_Recovery_Options#Factory_reset.
For more details on Hotspot configuration, please refer to this page: https://wiki.teltonika-networks.com/view/RUT240_Hotspot.