[2020] Can't frag on ground targets.

Most probably you see them destroyed be your AI allies. On majority of maps there are AI tanks, artillery and in many cases AI CAS.

You need to have high calibre + proper ammo to destroy AI tanks and pillboxes.

It's hard to judge where is the problem, you need to be more specific. Best - give us link to server replay from battle. https://warthunder.com/en/tournament/replay/