Did you try pairing them one at a time (IE only keyboard on then pair it and only mouse on and pair it)?
On top of that, in order to use multiple BT devices at the same time your tablet must have a BT radio of the latest BT version 5 specs which supports up to 7 simultaneous connections.. Even though it is a "new" tablet, due to the low price points of the ONN devices it may not have the newest BT hardware..
To make this even a bit more confusing, the possiblity exists that both your keyboard and mouse may be attempting to use the same BT profile causing a conflict and one may need to be set as "active"..
See explanations
HERE"Keyboard and Mouse
Keyboards and mice are the next most commonly-connected wireless devices. It makes sense to wonder if you can connect these two peripherals at the same time.
A Bluetooth keyboard and mouse can be connected at the same time. These two devices will pair similarly to other Bluetooth devices. While both share the same Bluetooth profile, Human Interface Device Profile (HID), the two do not interfere with each other and can be used simultaneously.
Tech Notes:
Typically used on laptops, a Bluetooth mouse and keyboard also work perfectly on smartphones giving users an “almost desktop” experience.
The Human Interface Device Profile (HID) is basically a wireless wrapper around the USB standard.
Bluetooth connectivity is backward compatible meaning older accessories will function when paired to newer host devices.
How To Connect Multiple Bluetooth Devices
Connecting multiple Bluetooth accessories to a single primary device is simple. However, you might want to have each device’s instructions on hand.
Power on your Bluetooth peripheral into a pairing mode, often requiring you to hold the power button for a few seconds or two buttons at the same time.
Find the Bluetooth icon on your laptop, tablet, or smartphone and toggle it to the on position.
Look for the new devices to show in the list of paired or available devices. Sometimes this is a sub-list or menu on the primary device.
Tap or click on the new device you wish to pair.
Some devices like keyboards will ask you to match enter or confirm a numeric sequence.
After following this general process for each of your Bluetooth devices, they should begin functioning concurrently.
Remember:
In the case of multiple Bluetooth devices with the same profile, you may need to set one as the active device."
