Trackpad still not connecting

I have an iMac 2013 running OS High Sierra 10.13.6.

Trackpad has worked fine since I bought the machine. Yesterday I changed from my regular rechargeable batteries to some Duracells. When I reconnected the trackpad it worked to start with but shaky and delayed pointer movement until it disconnected entirely.

I've followed the previous questions and solutions and tried:

- removing the battery cap, restarting, replacing battery cap, press and holding the on button but it never connects.

- resetting the PRAM and SMC.

- following the first step again removing battery cap etc

- replacing the batteries back to the original rechargeables that worked before as they have no fully charged.

- step 1 again.

Still no luck.

I can connect a wireless mouse with no problem.

When I go into Bluetooth settings and try deleting the trackpad, it magically reappears almost immediately. There is also no + button to try and add the trackpad that way.

Would love a solution! Thanks!