Let's Encrypt doesn't issue certificates for IP addresses.
ZeroSSL does, it was fairly straightforward to get a free one through their website when I tried many months ago. I'm not sure whether they make it available via their ACME integration though. When I tried, that wasn't available yet.
Nope, that feature has been "shelved". Please also see the following thread:
Also the only free ACME providing CA that issued certificates for IP address is down. Not sure how/why (it was a rather unknown Chinese CA).
Note that ZeroSSL does provide certificates for IP address, but just not through their ACME API. Only using their webinterface and/or REST API I believe (or that has changed too), but that method is subject to rather harsh limits, unless you pay. But I believe getting just one cert for the IP address using ZeroSSL should be possible.
Tried it a few days ago when I merged RFC 8738 into my Certbot forks "for own use" branch filled with some nice features (IMO) not getting in the main Certbot branch it was still not possible using ACME.