I have been troubleshooting my problem from a long time without success. I can describe the problem by the title only. The SNR drop from 30 to 18 becomes sudden then it goes up and down all the time. Ping plotter can measure PL and online games can also measure chokes and PL. I should mention the whole bandwidth is just 512/2048 up/down KBit so even 10 SNR should be fine. However I can't say that how SNR goes because it may dip to 0 for less than 100 ms and none of devices will notice that, but it will just appear as a PL or something like that. In some rare cases I lose DSL connection completely and my ADSL modem again will reconnect in moments. Sometimes CRC appears on upload and downlod stream and it goes away if I just rebooted the DSL modem or it will become fine again after moments then goes hell again. I checked my line and wires seems to be connected properly to my modem and to the local box and I feel like I can't do anything. I have a trash local service to support me well on that. What could cause this issue?
That might be caused by RF interference, and that is notoriously hard to debug. What kind of modem are you using, one that runs OpenWrt by any chance?
Usually the providers will test their lines up to the box at the entrance of your house. From the box up to the sockets of the apartment or the house they say that it's your problem.
Therefore you could temporarily connect the modem there and verify that the problems still occur, so you can open a trouble ticket to your provider to check your line.
Trying a different modem would also be a good idea, to rule out a faulty device.
Which device are we talking about?
What is the power supply: Battery with nothing else attached than this device, or the inverter you mentioned before?