I found that 19.07.0-rc2 is avalible and I only run 18.6.02. When I look in https://downloads.openwrt.org/releases/19.07.0-rc2/targets/mediatek/, I dont see my hardware (MT7621) but I see MT7622 and MT7623. Is it possible to install either of these on my router or are the newer not compatible with my MT7621 system (Netgear R6220)?
you should look for mt7621 device under ramips folder
But I run a MediaTek MT7621 system, Not a Ramips MT7621 system, Unless Ramips is a new name for MediaTek.
MediaTek folder only holds ARM MediaTek SoCs, MISP based architecture are made by Ralink (which bought by mediatek later) so the name ramips created, ra(link)+mips,