Resolution change on a 4k TV

Hello everyone,
I’m new to the forum, but I’ve been using Volumio for 2 years now. I’m having an issue with screen resolution. I’ve installed Volumio on a Raspberry Pi 5 and I am using the HDMI output to connect to a LG 4K TV. Everything works perfectly, but I need to reduce the resolution to 1920x1080. I tried modifying the userconfig.txt file by adding the lines hdmi_group=1 and hdmi_mode=16 (I also tried other modes) but the resolution stays at 4K and I can’t force it to full HD. Is there anything else I can do? Thanks to anyone who can help.