Assuming you’ve got a nice fresh certificate from LetsEncrypt and are in the directory where it, your key, and the LE root certificate1 (TrustID X3) lives:

cat ca.cer dst.cer > fullchain-plus-root.cer
openssl pkcs12 -export -in fullchain-plus-root.cer -inkey -out domain-fullchain-key.p12 -clcerts -passout pass:wlc
openssl pkcs12 -in domain-fullchain-key.p12 -out domain-fullchain-key.pem -passin pass:wlc -passout pass:wlc
cp -a domain-fullchain-key.pem /tftpboot/certs/
transfer download mode tftp
transfer download datatype webauthcert
transfer download serverip 
transfer download path /certs/
transfer download filename domain-fullchain-key.pem
transfer download certpassword wlc
debug pm pki enable
transfer download start

Check the logs to make sure the import was successful. If so you will be reminded to reboot. If so:

reset system