Registering with the UIF and paying over contributions is the employer's legal duty, not yours. Failing to do it is an offence under the Unemployment Insurance Contributions Act, and the Fund can recover the unpaid contributions from the employer along with penalties and interest.
That does not automatically produce a benefit for you, because the Fund works from declared employment records. What helps is evidence: payslips showing UIF deducted, bank statements showing the salary arriving, a written contract, or anything else establishing that you worked there and for how long. If UIF was deducted from your pay but never paid over, say so explicitly when you claim, because that is a different and more serious situation than never having been registered at all.
Take it to a labour centre in person rather than trying to resolve it online. A claim with a missing employer record needs someone to look at your documents, which the portal cannot do.
Be aware that the UIF's online systems have had a difficult few years, including a lengthy outage and an old portal that kept appearing in search results after it stopped being valid for claims. If you are submitting online, check you are on the current uFiling site and not one you reached from an old bookmark.