Update on this scenario...
Contacted ASUS Support moments ago, and according to them, for Guest Network Pro to work, all routers/nodes must be one of the following models:
GT-AX6000, GT-BE19000, GT-BE96, GT-BE98, GT-BE98 Pro, RT-AX57 Go, RT-AX86U Pro, RT-AX88U Pro, RT-BE86U, RT-BE88U, RT-BE96U...