Using 6S with phone feature on 8 Plus

I have an 8 plus that has cellular. I have an iphone 6S with an unactivated sim card that I want to use with the feature on the iphone 8 plus that says: (under Phone setting - Calls on other Devices) -

Use your iphone cellular account to make and receive calls on devices signed into your icloud account when they are nearby and on Wifi.

Both are on same wifi network. Both have each other turned on under this feature. Both on same icloud account.

Imessages and facetime will not activate on 6S. (I have tried both with the sim card in and out.)

Even though the imessage and facetime won't activate (wait 24 hours and still get activation error, date time are correct), I am able to imessage and facetime to someone with an iphone, but sms to someone with a different phone does not work. Making a phone call does not work at all.

After hours of discussions with apple support, I have learned that the software on the iphone 6S is different (than that on an ipad which works fine) and it is not considered a 'device' on the 'calls on other devices' feature. This is confusing at best, as i consider my iphone 6S to be a device, and it is listed as one of my devices under the feature.

I thought maybe if I hotspot or bluetooth from 8 plus to the 6S, i could temorarily give it time to activate the 6S but this didn't work. I don't think there is a work around for this issue so I thought I would ask the communities to see if anyone found a work around until apple can fix this confusing problem. I just want to share my celluar data so I can use my iphone 6S as a second phone in my house.

Any thoughts?