Hi Mauricio,
Are you still experiencing the issue of being unable to sign into a 10.4 portal from iOS device with 12.1.1? What version of the field app are you using? I tested this with our 10.4 Portal and a iOS device with 12.1.1 and 12.1.2 with app 3.1 and both working as expected. Can you try upgrade the iOS device to 12.1.2 and try again?
Also, are you able to sign into this Portal from other devices such as Android or Windows, or other iOS devices?
And can you provide more details on your Portal setup, what type of security and authentication, do you use VPN or is it publicly facing, does it work on internal network, etc?
Regards,
Phil.