Try separating the two Fives as a stereo pair and unplugging the Fives and Sub from power for a couple of minutes. Then reboot your router. Swap the position of the two Fives then plug everything back in. Wait until all speakers are connected to the network, then create a stereo pair with the two Fives. Then add the Sub to the Fives and test again.
Beyond that good advice, there’s the possibility of ‘simple’ wifi interference causing the Sub to not latch on to the signal of the system.
Hi @Diegoo2000
Welcome to the Sonos Community!
You’ve got your Sub in a separate room and grouped with your Fives.
Instead, you want to “bond” the Sub to the pair of Fives, so all 3 products are in one “room”.
In the Sonos app, select Settings » System » Studio » Add Sub and follow the on-screen prompts.
I hope this helps.