Most probably you have your devices with mac random address enabled on your main wifi connection settings, this means that when they connect to the deco they are seen as a new device, and a new IP is given eating, slowly, all the dhcp pool available. The guest network seems to not save anything, and thus the IP pool is not eaten. Check your offline devices, delete them from the Deco app and go thru all your home devices and disable the mac address randomization for your main network. And only allow your "home" devices connect to the main network, everybody else (visits, other non-resident family, should only be allowed to connect to the guest).
I had a couple of android mobiles here that had a nasty android 11 bug that would cause any wifi to always use random mac addresses, before getting rid of them, I had to manually, one each month, to go to the disconnected devices and delete the older ones to avoid them eating that IP pool. And by then I asked here that tp-link they should add an expiration to the IP pool reservation list, that we could select (like forget devices that did not connect in the past week or month for example), thus freeing back these IPs to the pool.
 

Thanks guys. For some reason, resetting the Deco didn't work, but changing the SSID did. I don't know if the randomized MAC setting was the culprit, but I disabled that feature on my phone. The rest of my devices unfortunately don't have such an option available, nor do I know if they use a fixed MAC address. 

I did not find changing the SSID as a solution during my Googling, so hopefully this thread will prove helpful to someone in the future.