Боролся в основном с помощью пакетика силикагеля внутри корпуса + тщательная просушка и продувка корпуса горячим воздухом перед закрытием корпуса.
Материал – обычный FR4, он на 1.2 ещё в целом более-менее работает. Не идеально конечно, но взять условный Rogers тогда не было возможности.

Got it, thanks! I overlooked!
It looks like an SMD transistor or microcircuit.)
Well, that means good, so I can order LNB with a filter again on Amazon! )

I was shocked that the device came mechanically broken.

After that, I opened my filter to make sure there was something in it.

Here’s what a 1420MHz passive filter looks like from the inside with Aliexpress.
There the filter is immediately visible! 🙂

https://ibb.co/NFFvqRH

Hello. I believe that the IC3 component is LTCC filter. So everything should be fine here.

I ordered it because of the good customer reviews.

He came to me with a broken shielded cover.

I already wanted to return it with a request to exchange it for the same one, but undamaged, but I saw that there was no filter under the lid!

It’s a simple broadband LNB!

I phoned Amazon support service, they said that the seller is not they, they have a warehouse.

Probably someone bought it before me, opened the lid and saw that there was no filter, returned it back to the warehouse, and they sent it to me!)

I returned it back to Amazon and get your money back.
Amazon is a very reliable platform, they are always correct with buyers for all problems!

Here is a photo of what I received.

https://ibb.co/3FtgW0x
https://ibb.co/TRwwyk8
https://ibb.co/pbR8LDJ

Perhaps such information will be useful to someone.
I do not understand way this device without a 1420 MHz filter has good customer reviews.
Lovers of radio astronomy probably did not check this device with measuring instruments, for the presence of a filter in it.
I would, too, never opened it and checked it.
It’s good that I got it with the lid open.