Struggling with UPC Connect Box Router and TP-LINK TL-WA850RE Amplifier Synchronization

Hello,
I have been struggling with the synchronization of the router (UPC - Connect Box) and the signal amplifier (TP-LINK TL-WA850RE) for a good few hours. As I am a bit off the beat with this type of topic, I will try to describe it very "in a pathological way". Therefore, please forgive all kinds of shortcomings - especially word-formation - on my part. Until now, I had a WI-FI adapter connected to the desktop, through which I connected to the main router. However, due to the quality of the connection (only on a desktop computer) I decided to get the amplifier mentioned. Here, however, a problem arises. Alternatively: a lot of problems.

I disconnected the old adapter and connected the desktop computer with a new purchase cable. Unfortunately, contrary to the manufacturer's assurances, I can't find the TP-Link_Extender / TP-Link_Extender_XXX network at all (even after repeated attempts to 'press' both devices). Immediately, however, connects me with the current one and, more interestingly, completely blocks the possibility of previewing / changing the others. Just after clicking "connect to a network", a message appears saying that "Windows (Vista - to be precise) cannot find additional networks". Also - I can't even go to tplinkrepeater.net/192.168.0.254 (the site is unreachable). However, I can enter the configuration of the main router. I checked the TCP / IP parameters (knowledge acquired through googling) and even tried to change something there, but apparently it didn't help much. At the same time - when I disconnect the Internet, I can enter the configuration of the TP-Link repeater via 192.168.0.254, but when I select the main network, it does not show mine, so I can't even finish it ...

When I tried to connect a desktop computer to the main router - I had about 90 Mb / s. When I have a cable connected from the amplifier, the speed is around 50 Mb / s. On the old adapter it was about 10 Mb / s. Someone could advise me on this matter? Explain why suddenly I do not have the option "disconnect from the network" or the possibility of connecting to another? I do not want to believe that this amplifier IS ALREADY working, despite the fact that it does not detect any new network and did not make any configuration. Especially that in all the guides it looked different than it did for me.

I will be very grateful for your help.

You don't see the extra network because you probably didn't configure it on the amplifier and it just extends the old name (SSID) from the main router. To be able to manage the amplifier freely, it is good to change its addressing from smart IP to Static IP by permanently entering the IP address from the main router pool. As for speed, the amplifiers are also fine, they cut the speed by half. One radio must pick up and then send a signal.

I changed this address according to some guide, entering the following values (in the IP address I entered the value from IPv4 IP address). I restarted the computer without any changes.
192.168.0.254 - the site is still unreachable and the following message pops up at tplinkrepeater.net (picture two). I am to understand that I should disconnect the amplifier cable from the computer, connect the current adapter (to have the Internet again) and then configure anything?

I wrote about addressing on the amplifier but ok you will need it from the beginning.
Reset the amplifier to factory settings (keep the RESET button pressed for more than 10 seconds) then connect to the computer by cable and after entering what you have in the picture into the wired card you must log in to http://192.168.0.254 of course the wireless connection must be disconnected. You do not need internet access to configure the router. If you have access to the amplifier, you configure (change) the IP address to one from the subnet of the main router (you need to check what DHCP is distributing there) and configure a wireless connection with the main router.

I will present my actions, step by step.
1. I disconnected the wireless connection.
2. I reset the amplifier and connected it to the computer with a cable.
3. Through 'network and sharing center' -> 'view status'' -> 'protocol (...) 4' -> properties I changed the IP address (to the one previously), gateway and DNS server for a new network; mask, of course, automat
4. I searched for the lost Extender network and connected to it.
5. I logged in to 192.168.0.254 and, in fact, there was a possibility of configuration (without the implementation of point 4 it was unreal). I could even "hook up" to the main network!
6. ... my enthusiasm is gone (screen).

When the amplifier reset again to "catch" the changes I made, I was unable to connect to ANY network (at least for the Internet to work). Both with the hybrid and the current one with UPC. I also tried to press this magic button on the router and on the amplifier itself. Unsuccessfully. After clicking on 'Settings' (next to the 'Quick configuration' bar), a message about the lack of connection appears. However, when I mark - a bit in spite of the actual state of affairs - a link about having an extended network, it immediately redirects me to the tplinkrepeater.net website (with the same message as in the previous post). Would you be kind enough to correct my mistakes? I would also be grateful if you could simplify the following fragment of the message: "(...) you change the IP address to one from the subnet of the main router (you need to check what DHCP is distributing there) and configure a wireless connection with the main router ''.

IP. wrote:

When the amplifier reset again to "catch" the changes I made, I was unable to connect to ANY network (at least for Internet to work)

Have you returned to automatic settings on the network chat? Because you have addresses rigidly written !?