I have 3 WAX615 and I am trying to configure mesh without Inisight

Not aware having spotted this should be feasible at all. All the documentation available eg. with the WAX615 Data Sheet and the Insight Managed WiFi 6 AX3000 Access Point Model WAX615 User Manual does always cover an Insight Instant Mesh - but no manual configuration of such a free Instant Mesh.

Definitively, if the customer does expect seamless roaming using WPA3-Personal (WPA3-SAE) he needs to understand that this requires either some controller- or cloud-based assistance.

For my part, I'm buffed people desperately want this considering some cabling and a capable PoE+ switch with 2.5 GbE would provide a much better service. Minimum starting point would be a MS108TUP allowing full Insight management, hardcore network admins might go for a MS108EUP. Last but not least, a dedicated WiFi backhaul offers the best of both worlds with easy setup and great performance, is simple to set-up, and will perform over all of its life time.

Before your customer does try: No, I'm not Netgear, just an experienced networks and systems admin deploying such networks, some bigger sites like an ice hockey venue for some 2500 people, among bunches of smaller deployments with several hundred WAX6xx and MS510TXUP on a bigger 10 GbE backhaul where required. 

Curious what this RAX43 should do in this customer environment.