jan_kon wrote: Ad1. Yes, a modem in the bedroom to be exact. Can I receive TV here if there was no cover?
Yes, this should be possible as long as there is full signal in the UPC cable. Except that you would need to do a few things.
What you have on the wall is probably not an RTV socket, just a cover and a cable grommet (removing the cover would make it clearer). In such a case, a signal splitter for TV and IP is installed between this "socket" and the Internet modem. And that's it.
Looking at the photo from the first post, it looks like such a splitter is mounted in a box in the apartment. This is also done by splitting the signal so that it can be delivered to different rooms. You would need to check whether it is actually a UPC installation (I think you can see the "UPC" marking on the cable going to the splitter). You can check this by disconnecting the second cable in the box from this photo - the one with yellow bands on the connectors.
If the internet doesn't go out, it means it's going through the blue splitter in the box. Then all you need to do is move the splitter to the bedroom and install it between the "socket" and the modem.
However, formally it should be done by UPC.
jan_kon wrote: Ad2. There is neither TV nor Internet in the second nest. The signal in the additional cable is very weak. It searches for a few channels but they don't receive them. Can anything be done about it?
This means that it is confirmed that the socket itself is disconnected from the cable in the wall, and the cable itself is probably not connected to anything at the other end, and what the TV or DVB-T decoder receives are signals from the cable itself, working as an antenna... ;)
Of course, there is also a slight chance that the cable is connected to AIZ, but something is damaged in the installation or connection. But we won't solve it here - it requires looking into the telecommunications shaft or the cupboard on the first floor.
Ask the administration about AIZ, because it is possible that when installing the Internet, UPC disconnected you from AIZ, thinking about connecting all the sockets in the apartment to their network, but they did not do it for some reason.