Hello @agustinsivoplas
If the wall is simple wood-framed hollow construction with sheet rock, it won't reduce signal strength by much. However, if it's solid (e.g., concrete or brick) or covered with plaster (especially plaster with metal lath), it will reduce signal strength significantly. Dealing with this can be challenging.
You may be able to get better performance by moving the primary into a more central location where it isn't having to send WiFi through that wall.
Other than that, running Ethernet between the rooms can at least provide wired devices in the other room with better performance (connect to the LAN port on the primary Nest WiFi Pro unit). In addition, once Ethernet has been run, another Nest WiFi Pro unit can be connected to that Ethernet (add an unmanaged switch to make more ports available). That allows it to provide WiFi to devices in that room, but carry their traffic back to the primary via Ethernet instead of trying to go through the problematic wall.
If running Ethernet is not an option, then you might be able to add one more Nest WiFi Pro unit as a secondary. This will only help if it can be placed in a spot where it has line-of-sight to the primary and through that door into the other room.
Basically, with all of these solutions, we're trying to avoid trying to push WiFi signals through a wall that is (apparently) attenuating them substantially.
