Whooohoo! Got it working this morning (with hangover and everything). Thanks for the help
@PBCool but seems I didn't actually need help

Learned a lot in the process so I think I finally know how this whole Fibre provider, CPE & ISP thing now work.
In short (based on MY setup, can't comment on other providers with other devices etc.):
So this is all purely speculation on my side so don't nuke me if I get it wrong...
The so called CPE "activation" that the Fibre providers (SADV in this case) does physically to the device (my Calix 813G-2 in this case) doesn't actually do anything important even though it seems that way. It's just a batch of post setup commands send to the device to lock it down and prevent the customer from being confused (by disabling it's wifi) or messing with it and also switches on the service light to indicate they know who the MAC address etc. belongs to and they can see you on the network. So most of the magic is happening on their side.
The CPE (ONT) activation or provisioning you mention is vital. SADV will have an Calix E7,etc on the headend (OLT), when you plug your 813G-2 into the PON network the provisioning happens as follows on Calix:
-ONT is discovered on the PON, Broadband light will be on
-ONT checks, then downloads the latest firmware from the OLT to your 813G-2 and reboots, etc.
-ONT will be provisioned by SADV, most importantly assigning a VLAN to the ONT (in your case CI VLAN on SADV)
-Other settings such as WIFI, FXO VOIP,Bridge or residential gateway mode, will also be setup
The "service" light that everyone always mentions needs to be on is in fact just a dumb light that's locally or remotely controlled on the unit. In fact, you could probably SSH into the device and switch the light on or off yourself if you google the commands for it or related routers. I think the idea is for the providers to have a way of indicating whether everything is fine on their side, and possibly also switch it off remotely if they're planning maintenance on the network etc. (although, I doubt any provider actually bothers with that).
The service light will be on when the ONT has been provisioned by SADV and a data service is assigned to it, in your case probably a VLAN assigned to CI
Now, if you reset the CPE device then all the default settings return (wifi on, service light off etc.) but that doesn't mean the device cannot connect anymore, you're still good to go on the fiber network IF they've "activated" your device previously, if everything is still fine on the fiber provider side and your ISP is activated and linked to the device then you're good to go... with or without the service light.
Ill be surprised if you could connect to the internet without the service light active