autossh : remote side connectivity fails while ssh tunnel still active

I'm trying to keep a tunnel using autossh, with the following command :

autossh -M 0 -o "ServerAliveInterval 30" -o "ServerAliveCountMax 3" -R 0.0.0.0:8865:localhost:8765 user@remote

The goal is to make Client Box app on a specific port (a motioneye camera setup) viewable from internet. (4G internet connection is not suitable for NAT forwarding, so a tunnel seems to be appropriate)

                  ┌───────────────────────────────────────────────────────────┐
                  │                                                           │
                  │                                                           │
┌─────────────────┼────┐                      ┌───────────────┐      ┌────────┴───────┐
│                 │    │  ┌──────────┐        │               ├──────┤   remote box   │
│     Client Box  │    ├──┤  4G box  ├────────┤    INTERNET   │      │8865 public port│
│  init ssh tunnel│    │  └──────────┘        │               │      └────────▲───────┘
│                 │    │                      └───────┬───────┘               │
│  ┌──────────────▼─┐  │                              │                       │
│  │  app on 8865   │  │                              │                       │
│  └────────────────┘  │                              │                       │
│                      │                              │                       │
└──────────────────────┘                              │                       │
                                          ┌───────────┴──────────┐            │
                                          │     Test box         │            │
                                          │                      ├────────────┘
                                          │  browser remote:8865 │
                                          │                      │
                                          └──────────────────────┘

This setup works, I'm able to connect to the App on Client Box through Remote box tunnel endpoint.

But, after 12 hours or so, it is not reacheable anymore... But the ssh tunnel is still active from the Client Box side !

If I reboot Remote box, it is working again (autossh is doing its job correctly).

Is there a way to debug this ?

Before rebooting, using netstat on remote, there's a bunch of CLOSE_WAIT connections (all ip's are anonymized) :

user@remote:~$ netstat -taupel

Proto Recv-Q Send-Q Local Address           Foreign Address         State       User       Inode
tcp        0      0 0.0.0.0:8822            0.0.0.0:*               LISTEN      marc       14443
tcp        0      0 0.0.0.0:ssh             0.0.0.0:*               LISTEN      root       13018
tcp        0      0 0.0.0.0:8865            0.0.0.0:*               LISTEN      marc       14996
tcp        0      0 99.ip-99-99-99:8865     77-77-77-77.cou:53909   ESTABLISHED marc       269229
tcp        1      0 99.ip-99-99-99:8865     88.88.88.88:4689        CLOSE_WAIT  marc       273524
tcp      517      0 99.ip-99-99-99:8865     88.88.88.88:62618       CLOSE_WAIT  marc       270519
tcp        1      0 99.ip-99-99-99:8865     88.88.88.88:4742        CLOSE_WAIT  marc       273702
tcp      530      0 99.ip-99-99-99:8865     88.88.88.88:4712        CLOSE_WAIT  marc       273700
tcp        0    628 99.ip-99-99-99.:ssh     88.88.88.88:4867        ESTABLISHED root       273951
tcp      517      0 99.ip-99-99-99:8865     88.88.88.88:63033       CLOSE_WAIT  marc       270489
tcp      517      0 99.ip-99-99-99:8865     88.88.88.88:62505       CLOSE_WAIT  marc       269762
tcp      516      0 99.ip-99-99-99:8865     88.88.88.88:62850       ESTABLISHED marc       270756
tcp        0      0 99.ip-99-99-99.:ssh     dsl-66-66-66-66:45052   ESTABLISHED root       274146
tcp      517      0 99.ip-99-99-99:8865     88.88.88.88:62504       CLOSE_WAIT  marc       269763
tcp      516      0 99.ip-99-99-99:8865     88.88.88.88:4747        ESTABLISHED marc       273701
tcp        1      0 99.ip-99-99-99:8865     88.88.88.88:4761        CLOSE_WAIT  marc       273703
tcp        0   1400 99.ip-99-99-99.:ssh     77-77-77-77.cou:53649   ESTABLISHED root       14909
tcp        0      0 99.ip-99-99-99.:ssh     88.88.88.88:62552       ESTABLISHED root       269900
tcp        0      0 99.ip-99-99-99:8865     77-77-77-77.cou:53641   ESTABLISHED marc       269231
tcp      590      0 99.ip-99-99-99:8865     88.88.88.88:4690        ESTABLISHED marc       273523
tcp      517      0 99.ip-99-99-99:8865     88.88.88.88:4675        CLOSE_WAIT  marc       273699
tcp        0      0 99.ip-99-99-99.:ssh     66.66.66.66:56662       ESTABLISHED root       274175
tcp        0      0 99.ip-99-99-99.:ssh     55.55.55.55:62864       TIME_WAIT   root       0    
tcp        0  32768 99.ip-99-99-99:8865     77-77-77-77.cou:53911   ESTABLISHED marc       269230
tcp        0      0 99.ip-99-99-99.:ssh     88.88.88.88:62520       ESTABLISHED root       269764
tcp        0    100 99.ip-99-99-99.:ssh     77-77-77-77.cou:53569   ESTABLISHED root       14023
tcp      517      0 99.ip-99-99-99:8865     88.88.88.88:62619       CLOSE_WAIT  marc       270518
tcp        0      0 99.ip-99-99-99.:ssh     55.55.55.55:21735       ESTABLISHED root       273543
tcp      517      0 99.ip-99-99-99:8865     88.88.88.88:62798       CLOSE_WAIT  marc       270103
tcp6       0      0 [::]:8822               [::]:*                  LISTEN      marc       14446
tcp6       0      0 [::]:ssh                [::]:*                  LISTEN      root       13029
tcp6       0      0 [::]:8865               [::]:*                  LISTEN      marc       14997

Thanks !