Your tag should beep when you travel through a toll point. If your tag doesn't beep, you'll be charged additional fees each time you travel. To figure out how to avoid this, take the health check.
If you've removed your tag on purpose, you can still travel, but you'll be charged an additional fee each time you travel.
Check the installation guides here.
To order a holder:
Order a replacement tag
Transurban Linkt and Linkt are trade marks of Transurban Limited ABN 96 098 143 410