Well, the IRS says this (pre Social Distancing apparently):
What happens if I cannot verify?
If we can’t verify your identity online or over the phone, we will ask you to schedule an appointment and bring the documents listed above to your local IRS office to verify in person.
Understanding Your Letter 5071C or 6331C | Internal Revenue Service (irs.gov)
I suggest you contact the Tax Advocate for advice.
Taxpayer Advocate | Internal Revenue Service (irs.gov)
