Citizenship for UK born child

Greetings to all:
I have been on Tier 2 WV for 5 years and I just got my ILR. My child born in the UK 2020. Because at the time of birth I had TTier 2 visa and since my child would entitle to NHS services only for the 1st three months we applied for BRP for my child, for this I used a Travel document issued by my embassy in London( expired Nov 2020) The process for passport application from my embassy can take up to 6 months and I want to register my child as British Citizen now not later.
When I start filling the application form ( Register child under 18 as British citizen) I tried to fill in the details for the travel document but the system doesn't accept it as its expiry date is in the past. Should I put No doe travel document and then try to explain on the next page why I didn't provide any details for the PASSPORT/TRAVEL Document?
Also, it didn't ask me about the details for the Birth Certificate? or for the BRP if any?
Should I just include a cover letter and upload all these as proof of my child ID?

I am sorry but I tried to explain as much so my Q is clear.

Thank you very much for all the help.