Full disclosure: I’m not a TCL user.

For the TV’s that I’ve encountered there is an option to use the TV’s remove to control an A/V system via IR. After selecting this TV option, you’ll need to train AMP to respond to the IR commands. You’ll typically need to declare a A/V system model number to the TV’s remote. Specify an arbitrary model that is not nearby.

Have you tried to just power off the TV and the Amp? Is HDMI-CEC on on the TV?

There is second dimension to this potential issue: If the TV and ARC are connected via an HDMI cable to the TV’s HDMI-ARC port, digital volume commands will usually be sent over the HDMI cable. Sometimes there is a TV option that can block this. The sneaky issue is that, while the TV remote, and even Volume buttons on the TV, can control ARC’s Volume while the TV is ON. Volume commands will not be sent from the TV when the TV is OFF and ARC is playing music. The work around is to force the TV remote to send IR Volume commands to ARC regardless of the TV’s power state.

When everything is working, the TV, cable box, and other video sources can all control ARC’s Volume. Many times this is automatically set up with no user interaction. Unfortunately, the industry standards are very lax on exactly how this feature should be implemented and it can result in a mess. When there is a mess, it’s always the other guy’s fault.