Determining origin of SSH traffic being blocked

I have a simple SSH jump host sitting at a remote site. I could connect without issue for the first two weeks. But now initiating a session seems to fail *unless I use a proxy*. jump host ------- ISP A -------------------------- ISP B ---------- my workstation Port 55555 is properly forwarded through ISP A. The connection simply times out.

ssh -vvv -p 55555 XX.XX.XX.XX
OpenSSH_9.2p1 Debian-2+deb12u3, OpenSSL 3.0.15 3 Sep 2024
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: include /etc/ssh/ssh_config.d/*.conf matched no files
debug1: /etc/ssh/ssh_config line 21: Applying options for *
debug2: resolve_canonicalize: hostname XX.XX.XX.XX is address
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts' -> '/home/mansomean/.ssh/known_hosts'
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts2' -> '/home/mansomean/.ssh/known_hosts2'
debug3: ssh_connect_direct: entering
debug1: Connecting to XX.XX.XX.XX [XX.XX.XX.XX] port 55555.
debug3: set_sock_tos: set socket 3 IP_TOS 0x10
debug1: connect to address XX.XX.XX.XX port 55555: Connection timed out
ssh: connect to host XX.XX.XX.XX port 55555: Connection timed out

If I push it over a VPN, for example, the connection is immediately accepted. It is only accepted so long as does *not* originate from my public IP at ISP B. Is there a way I can determine whether ISP B (or A) is blocking this traffic? And why were there no connectivity issues in the first two weeks?