How to get ssh to a computer behind two routers?

Question

SSH

Actually a question in a subject. There is a computer on Debian, which connects to the network through two routers. How to access it from outside?
PS Question closed. I forwarded the ports first from the router to the router, and then further.

Answer 1

forward ports from router to router and then to computer

Answer 2

Does the router looking at the Internet has a permanent ipv4 and has access to routers?
Forward port 22 on each router.
Does the Internet-facing router have dynamic ipv4 and have access to routers?
Forward port 22 on each router, connect to some Dynamic DNS service in order to know the current address from the outside.
Is there no access to routers or does the Internet go through NAT (that is, does the router looking at the Internet has an internal IP provider)?
The tunnel from the computer to the external Internet will help.
To do this, a computer can connect somewhere via VPN and receive a permanent IP output.
Or you can set up an ipv6 tunnel on the computer, then SSH will be available outside at the ipv6 address (this is probably the easiest to do, but ipv6 must work correctly for clients to connect to the computer).