ADSL modems are dying like flies. What could be the reason?

Question

Electronics

The story is this: in the already not quite small office, four different modems from three different manufacturers have already died over the past month.
The first was some kind of Billion issued by the provider. First, the web interface stopped loading, then periodically the modem "fell off" (there is no ping, the lights are blinking, as if everything is fine), which was solved by rebooting, and one fine day it just stopped working.
The second was D-Link 2540U. This is the only one that was found in the nearest store as an urgent replacement. The symptoms are the same and in the same sequence, the modem worked for two weeks and died. Changed under warranty for the same, without asking a word. Until today, lay idle in stock.
The third was the Zyxel P660 RT3. As far as I know, this particular model is considered indestructible. After two weeks of use, he also died, but under different circumstances: at first, the Internet began to work disgustingly (ping to any Internet site resembled the random function: delay spread from 21 ms to 3000 ms, errors like "unable to find node", "name could not be resolved", etc.). They kicked the provider for a long time, but he shrugged and kept saying that everything was in order on the DNS server. A reboot solved the problem for a minute or so. This lasted three working days, after which the modem stopped establishing a PPoE connection, and the provider said something about the impossibility of establishing a mac address (what are they talking about? You can clearly see the modem's mac in the LAN) and about the need for a replacement.
The fourth was the D-Link 2540U, which was in stock. He worked for 20 minutes, after which the interface stopped loading, the modem stopped responding, by the evening he gave up just like all the previous ones.
Optical fiber to wait another 2 months minimum. There are no other available options for connecting to the Internet, except for adsl, in this office. Tomorrow they bring another Zyxel P660, in connection with which I am more than ever worried about the question: how to avoid damage to this modem as well?
A little about the environment of all four modems:
1) All four were included in one surge protector. Servers also work through the same socket, and, to be honest, they have been working for a long time and without any complaints.
2) All four were included in the local network through one switch. The previous switch died (I hope) of old age, hanging up the network with broadcast storms for a couple of days. There is only one similarity between the new and old switches: they are connected to the same outlet.
3) The telephone line has been removed from the PBX. No one has the slightest idea what and how this mini-PBX is configured and working.
4) The working conditions for all four were not the best - they stood on the switch, which is why sometimes they warmed up quite noticeably (according to the sensations of 45 degrees, they somehow didn’t think of measuring).
I would be grateful for any advice or versions.

Answer 1

According to the description, it looks like dying telephone ports on routers.
Those. on telephone noodles, too much voltage comes to the modem or something like that. Possibly static.
Our switches behaved in much the same way when lightning hit the cable.

Answer 2

I propose to carefully measure the voltage between the "grounds" / housings of the switch, surge protector and PBX.

Answer 3

ground everything
deal with the temperature
put an uninterruptible power supply

Answer 4

A short circuit is possible, you can try to compress it again, for starters, the cable that is responsible for the Internet

Answer 5

I'm more inclined to the fact that your PBX provides a lot of power to the port, you can put a UPS with phone sockets and then connect the modem

Answer 6

Match =)

Answer 7

Line splitter installed?