@Gavriel Try to make the connection only in 5GHz, redo the configuration but now connect the extender only to the 5GHz network and see if anything changes. 

  @Andrsouzaaa yes, now there's no internet on the RE450 neither on 5G nor on 2.4G.

I also rebooted the main router and i see in the main router's clients the RE450 is connected and not blocked.

Any other device that I connect to the main router via 5GHz works.

So my guess is still that somehow the RE450 blocks the sharing of 5G

  @Gavriel I think your RE450 may be having a manufacturing defect in , I recommend looking for Tp-Link support and checking it out, but first one more tip from me... set the 5GHz channel to 36 and test again, this will eliminate possible doubts. 

  @Andrsouzaaa  5GHz uses channel 40. Why do you think that can cause the problem? I do see the RE450 is connected to the main router.

  @Solla-topee 

stability:

1. The setup is this:
 - main router: it's place is the same since we moved in to the house. The router itself was changed a few months ago from Mi Wifi to Tp-Link, but with both routers the RE450 worked without any problems for more than a year / more than 2 months respectively

 - RE450: it's place is the same since I bought it in Oct 2020

 - My work room where I usually connect to the RE450 is the same since the covid started. I usually connect to the RE450 from both my laptop and my android phone.

2. I use Wifi Analyzer android app and speedtest quiet a lot (I had some internet problems that eventually caused me to swithc the main router/modem, though it didn't help, and thus I also changed ISP recently. However all these problems were not effecting RE450, it worked without any problems all the time) so I can tell you that the position of RE450 is OK. In the same spot the signal strength of the main router in 5GHz is -78dBm

3. I changed now the main router to channel 36 (from 40) and it changed nothing (BTW: RE450 was using channel 36 all the time)

4. both the 5G and the 2.4G leds on the RE450 are solid blue. Manually setting the RE450 DHCP disn't change a thing :(

5. As I wrote above: RE450 worked without any problem for a year with my previous main router, then for almost 3 months with the current main router. Since the problem started about a week ago it never worked again on 5GHz. 

I also tested this:

in RE450 settings > Wireless >
Connect to Network: 2.4: enabled, 5: disabled, and Extended Network: both enabled => connecting to RE 2.4G or 5G: both have internet

Connect to Network: 2.4: enabled, 5: enabled, and Extended Network: both enabled => connecting to RE on 5G=> no internet, 2.4G=>has internet

Connect to Network: 2.4: disabled, 5: enabled, and Extended Network: both enabled => connecting to RE 2.4G or 5G: both don't have internet

  @Solla-topee 

actually after I wrote the previous comment I discovered that's something ever stranger:

Connect to Network: 2.4: enabled, 5: disabled, and Extended Network: both enabled =>
   Android phone connecting to RE 2.4G or 5G: both have internet

   Laptop connecting to RE 2.4G and 5G: both don't have internet

Again I used this laptop from the same place connected to RE450 on 5G for 80% of the time, and on 2.4G for 20% of the time for more than a year...

I also rebooted both the RE450 and my laptop, and still the same.

  @Gavriel Well, from my experience with Tp-Link products for years as a consumer and as a computer engineer, I'm sorry to say more to myself that your RE450 device is defective in the 5GHz band co-processor is having problems, if it is under warranty I recommend contacting Tp-Link and trying to replace it with a new one, it is not worth insisting on this device,and this is normal, I've already lost two RE450's, but I live in a tropical region that has high temperatures and that over time has affected my device, I don't know if it's your case, but it may have something else happened, I feel on my new RE450V4 that it heats up a lot, but so far it's working normally, and my advice is this, if it's under warranty, try replacing the s  your RE450 with Tp-Link, after all, warranty is for that.