I guess Dell Support Team full filled or limited about this issue. They couldn't answer queries yet. So, i have also same problem. I checked existing SSD 32GB drive in my Dell. So, it's SK hynix SC215 M.2 NVMe drive. It was working very well when i initially installed my Dell inspiron 7459. Then, i took as reference this existing SSD then purchased new one to extend SSD capacity to 240GB. I got Toshiba RC 100 240 GB PCIe (NVMe) SSD (that i can only find in store). I replaced the SSD M.2 drives with Toshiba RC 100 but it's not working. So, i'm trying to understand, is that M.2 B+M slot supports PCIe (NVMe) or not? I'm on my search in internet until 6 hours but nothing to find it. How Dell could not answer this simple question?
The SK hynix SC215 is a SATA SSD in M.2 format. I haven't seen any Inspiron 7459's running NVMe SSDs.
In addition to this thread, I did see two systems on UserBenchmark.com running with a Transcend SATA M.2 22x42 SSD here (TS256GMTS400S 256GB) and here (TS128GMTS400S 128GB). One of them has the Transcend as the only drive in the system, so seems it works as a boot drive. I believe the Transcend also has a 512GB capacity option.