Remote access to Linux graphical session with opensource software behind router without the need for PAT/NAT configuration

I use anydesk on my Linux behind my ISP router and my Dad PC also using anydesk on Linux behind his ISP router. I'd like to use an opensource software to gain remote access to the Linux graphical session that does not need complicated network setting (especially on the remote side) like anydesk. EDIT0: I want to know how Anydesk works so I can find a software that can traverse NAT routers without the need for "touching" the remote side ISP's router settings. Do you know an opensource software that can do that ?