Attach to the tmux session over SSH to continue your work.
You will receive an error the first time you try to reattach to existing tmux session.
$ ssh 192.0.2.125 tmux attach open terminal failed: not a terminal
The solution is to simply force pseudo-terminal allocation.
$ ssh -t 192.0.2.125 tmux attach [...]
Define RequestTTY in OpenSSH SSH client configuration file to make this permanent.
$ cat ~/.ssh/config
Host 192.0.2.125 RequestTTY yes
A will give you a tip, use -d parameter to detach any other clients attached to the session. It will ensure that tmux session window will use all the available space.
$ ssh -t 192.0.2.125 tmux attach -d [...]
Single or multiple jump hops are not an issue as you need to request a TTY on the target host only.
$ ssh -A -J milosz@192.0.2.2 192.0.2.125 tmux ls 0: 1 windows (created Sun Nov 3 22:54:40 2019) [179x63] (attached) Connection to 192.0.2.125 closed
$ ssh -A -J milosz@192.0.2.2 192.0.2.125 tmux attach -t 0 -d [...]
Additional notes
Excerpt from the manual page.
RequestTTY
Specifies whether to request a pseudo-tty for the session. The argument may be one of: no (never request a TTY), yes (always request a TTY when standard input is a TTY), force (always request a TTY) or auto (request a TTY when opening a login session). This option mirrors the -t and -T flags for ssh(1).