The A4Tech FB35C is a dual-mode mouse supporting Bluetooth and 2.4 GHz wireless connectivity.
A4Tech specifies that it can connect to up to three devices: two through Bluetooth and one through the 2.4 GHz receiver. It is listed as compatible with Mac. (A4Tech)
Because the FB35C does not appear under System Settings >> Bluetooth >> Nearby Devices when you attempt to pair it, the immediate problem is Bluetooth discovery rather than mouse-button, scrolling, or pointer functionality.
Apple describes the Nearby Devices list as the location where a Bluetooth accessory that is ready to pair should appear. (Apple Support)
Before investigating macOS or third-party software, verify the FB35C's own pairing procedure.
A4Tech specifies two Bluetooth slots. For Device 1, briefly press the Bluetooth button until the blue indicator appears, then hold the Bluetooth button for approximately three seconds. The blue light should begin flashing slowly, indicating pairing mode. For Device 2, briefly press the Bluetooth button until the red indicator appears and then hold it for approximately three seconds; the red light should flash slowly.
The Mac should then search for the device named “A4 FB35C.” (A4Tech)
It is worth testing both Device 1 and Device 2, because they represent separate Bluetooth connection slots.
If one slot does not produce a discoverable device, try the other using the same procedure. Also make sure the mouse has sufficient charge before testing; A4Tech specifies a low-battery indication below 25%. (A4Tech)
If “A4 FB35C” still does not appear, test the mouse in the same pairing mode with another Bluetooth-capable device, such as an iPhone, iPad, or Windows PC.
If it appears there but not on the Mac, the result narrows the problem to the Mac side of the connection. If it does not appear on another device either, the investigation should concentrate on the mouse's Bluetooth pairing state or hardware.
If the FB35C is already present under My Devices on the Mac, forget that existing Bluetooth entry and attempt a new pairing.
Apple provides a Forget This Device option for previously connected Bluetooth accessories. (Apple Support)
The 2.4 GHz connection should also be tested with the supplied receiver, if available.
A working 2.4 GHz connection and an unsuccessful Bluetooth connection would provide a useful comparison between the mouse's two wireless connection methods.
It would not, by itself, identify the cause of the Bluetooth problem.
The fact that the same FB35C worked with this Mac before the macOS 27.0 update is relevant because the behaviour changed after the update.
However, that timing alone does not establish that macOS 27.0 contains an FB35C-specific Bluetooth defect.
Third-party software should be considered as a separate part of the investigation.
Once macOS recognizes the FB35C as a connected mouse, utilities such as LinearMouse can provide additional control over pointer behaviour and other mouse functions. LinearMouse, for example, supports device-specific configuration using the mouse's vendor and product identifiers. (GitHub)
A4Tech itself also currently lists FB35C/FB35CS in its software/download section.
Therefore, the manufacturer's current FB35C software should be checked rather than assuming that no A4Tech software exists.
However, the operating-system requirements of any downloaded utility should be verified before installation; the fact that software is listed for the FB35C does not by itself establish that a particular download is compatible with the current macOS version. (A4Tech)
The important distinction is that mouse utilities normally configure a mouse after macOS has recognized the device.
They should therefore not be presented as a guaranteed solution to a mouse that does not appear in Bluetooth discovery.
Their potential value is in making the FB35C function more fully or differently once the Bluetooth connection has been established.
