It should be possible to have DFU over Wi-Fi. NCS already has the FOTA download library for DFU over Cloud for our nRF91 series, which connects to cloud over LTE. However, the support for Wi-Fi might not have matured completely yet. I will check with our team on this and get back to you.
Unfortunately, we don't yet have a sample for DFU over Wi-Fi yet.
I have found FOTA over Wi-Fi is demonstrated in the AWS IoT and Azure FOTA samples. You can also find links to the Azure and AWS library documentation in the sample page.
It should also be possible to perform FOTA with an image on a generic server or nRF Cloud. However, we don't have a sample for that yet.
