downgrade 7841 ip phone

[theicewizard61300]

I am new to voip and am struggling.

I am trying to downgrade the firmware on my iphone from 12.5 to 11.0 because it is not compatible with my routers. Before I continue keep in mind I will not be connecting to the internet at any point in time during this process and I am using a single pacstar router which has 2 routable ports and 4 switch ports and the 7841s only use sip. 

I copied the firmware from a usb to my routers flash: I can see they are in there when I do dir | inc 78 (all the file names have 78 in them). I config vlan 40 for voice, and configured a sw as voice and a trunk port that allows all vlans. I configured dhcp, sip, tftp, domain name and hostname. The phone is able to pull an ip from my dhcp and I can ping the phone. When I restart the phone or reset it to default it shows it has an ip, it shows the routers hostname and domain name but then when I look at its status it says no dns. So even though i didn't think I needed a dns i configured one on the router and it's not getting the ip I set as the dns (I used the routers ip which the phone shows it has for the routers ip).

I tried doing the upgrade and create profile command but they say error file  deleted flash:/ (then some file that doesnt exist). My supervisor said that error message isn't really important though. But the phone is still not taking the new firmware and downgrading.

[Leo Laohoo]

Not all 78xx phones can support 11.0 firmware.  This will depend entirely on the HW revision of the phone.  

[theicewizard61300]

How can I check if the hardware does not support it. The specific file my 7841 has on it is sip78xx.12-8-1-0001-455.loads

And I need to downgrade it to sip78xx.12-0-1-11.loads

I also have all the related .sbn files

[theicewizard61300]

Update: 

I fat fingered an ip, I put the vlan as .226 and on dhcp I put for router to be .225

The phone took the firmware but it's not registering a phone number. Any must have commands I could be missing?