Yes, slogin (secure login) (ssh). I found a book from TLDP explaining slogin and ssh too. It's only a matter of dedicating some weeks, then. But is wonderful linux did it all by himself. I mean connecting A, B with the modem and A, B with each other given I could ping. When booting, I say.

i think, i prefer one with 3G ability thought USB modem.
TP-Link TL-MR3220
it is not more price, but have a good additional feature.
that command can undone with restart. after restart you have lost that changes

PS for login use "ssh 192.168.0.100"
for ocassionally file copy see the "scp" command - copy files from one machine to another via ssh.
for frequent files exchange i think, is be good to set up fileserver - on samba ( windows smb file sharing protocol ) or via nfs - linux native...

Thank you. It's the way I chose (restart). For the router, too late, but I would not miss USB. Too much complicated electronically and infinite complicated software, no matter how fast.

actually it only sounds complicated, but it would be just a matter of plugging in the 3g modem and inputting a username/password into the router's web based configuration.
that being said, i believe a decent speed cable modem is faster, 3g service tends to be more expensive, and the only real advantage of 3g is mobility, unless it's the only type of service available in your area.
either way, if you don't need it, then it's not worth paying for, no matter how cheap.

Thank you. By the way, how do I know the fully qualified hostname of my machine (FQHN)?

EDIT: I have just learned it.