You may use SPLA licenses on-premise. However, there is a mandatory condition. If that condition is not respected, using SPLA licenses on-premise is prohibited.
One of our clients had a mix of CSP and SPLA licenses. A reseller sold them SPLA licenses for SQL Server Enterprise and Windows Server Datacenter. We had to tell them that they were non-compliant.
Unfortunately, they could not blame the reseller. As the licensee, the end-user of licenses, you are responsible for compliance. Resellers often transact licenses without providing the necessary compliance advisory.
You may only use SPLA licences on-premises as a part of a service provided to you by an SPLA-authorised services provider. They must have access to the servers on which SPLA licenses are deployed. Moreover, they must be the principal administrators of such servers.
Otherwise, usage of SPLA licences on-premises is prohibited.
If you have doubts about your mixed licensing scenario, please get in touch with us using the form below, and let's look into your situation together.