Fairly certain once the device has been registered as activated for the first time on Roku's activation servers, the offer goes away.
Had a brand new Roku TV to activate as well.
I figured it may have been the DNS on my network.
So I used cellular IP on my iPhone w/ Safari (iOS 12.1, no add-ons, or ad-blockers).
So two issues.
2. The handoff on the redirect from Roku's offer page to Sling's website - at least on iOS Safari (WebKit).
I'm not claiming nefarious malfeasance, so let's set side intentions.
Seeing as how the offer is seemingly unable to be redeemed, it is tantamount to false advertising.
It would be difficult to include a physical promo code when selling older stock that's been lying around.
But then a $35 promo code should be digitally issued when the device is first activated.