Following @Andrew_Powell1 I would also suggest to try another cable.
Alternatively to your current settings in “/boot/config.txt” you could try
hdmi_group=1
hdmi_mode=96
for a 2160p resolution at 50Hz.
Regardless of that, I would recommend to put your settings into “/boot/userconfig.txt” because “/boot/config.txt” gets overwritten on Volumio updates.