Installing custom certs from letsencrypt ACME

Hi, I’m using nextcloud snap 31751 on Ubuntu server 22.04
I’m only using my LE certs locally, behind NAT so there’s no way I could use the letsencrypt feature of enabling SSL. Ive got a .ca .crt .key .pem files from the certbot host, shared over NFS so all I need to do is point my services there and they’ll automatically use the files.

Ive tried doing

nextcloud.enable-https custom

It asks for a cert, key, and chain file, and I do not know what those are supposed to be, ive tried to run it with a few variations but it gives me a syntax error on line 101 of ssl.conf and says cert.pem doesn’t exist…