The problem of access to the device connected to the "multi-modem". Unstable ping. What is the problem?

Question

network hardware

Good time! Already addressed here with this problem, perhaps I did not formulate the essence or did not indicate some details. In general, I describe everything in a new way.

On the "one" side:
There is a certain multimodem in which the sim card is installed - for the sim card its own ip address. We connect a switch to this modem with our own ip address of management, we also connect another host with our own ip to the switch.

On the "other" side:
I don't really know what is here, the same multimodem or some other piece of iron, but as a result, the data along the switch chain gets to the router.

The essence of the problem:
Everything seems to be working, but after some time, ping to hosts located behind the multimodem (switch, host) starts to disappear, and up to two ip addresses can disappear at once, or up to one can disappear. The nature of the disappearance is unstable and not clear.

The problem is solved by clearing the arp table on the router, but after a while it appears again.

Ping to sim card ip address is stable.

If anyone has had a similar problem or knows a solution, I'd be happy to help!

Answer 1

See the MTU value on the modem.