For two different projects I run Linux servers and experience a similar problem. After a few weeks, the systems don't accept any TCP connections from the outside. It's not possible to SSH to the system anymore and all other TCP connections are also denied. The system still responds to a ping.
One system runs Docker and three MongoDB containers. The other system runs Docker and five Minio containers. No other software is installed on these systems. Both systems run with an ISP, so without SSH I cannot investigate much.
Does anyone know where I should start to look?