In all the paperwork that you received, there's no 800 # for HR/Benefits for you to call w/ questions? I find that hard to believe. That paperwork is pretty much boilerplate content where they fill in fields from some DB that has your name, job title, manager's name, dollar amounts for this, that, and some other category, and the end date, the date to sign & return paperwork, etc. Other than the specific values, the letter is the same for every employee (by country) so the country specific HR contact info is in the boilerplate.
Reach out to someone you used to work w/ and ask them for the local 408 SJC number or google the Cisco public phone number that gets you an operator. From there, you can ask for HR/Benefits.
Yes, you're locked out and everything is now by phone and them sending your email updates to your personal email you provide to them when you open the case. You should be able to log into ADP externally once you're added to some list that doesn't require accessing it from a Cisco Trusted Device and using Duo to authenticate first.