After upgrading, the DVR, immediately, went offline for all the devices...
Do you mean you have a DVR that is linked to Deco m5 network and it shows offline on all devices after a firmware upgrade on Deco M5?
If it is, did you try to reboot the deco and DVR before? Please try this first.
Except the DVR device, how about your other devices like phone or computer, do they have internet access from Deco M5 now?
Could you please send a screenshot on offline page?
Thanks, good day!