Long story short, it looks to be a poor Wi-Fi implementation in the EVCS Charger ( Espressif32 module), that causes the abrupt WiFi disconnection. The ESP32 implementation does’nt appear to be fully compatible with smart Wi-Fi technology, such as Mesh or Wi-Fi 6.
Here is a drawing showing what is 
NOT working, in my case (The routeur is a mesh Wifi 6)
I have set up an additional Wi-Fi access, point dedicated solely to the EVCS terminal, few meters away from the charger, connected to Ethernet. The AP replaces the Wi-Fi router to communicate with the EVCS.
I am using a TP-Link TL-WR802N nano router (<€20), which is a 2.4 GHz only, single-band, very basic, non-smart router – the most basic I could find. If my assumptions are correct, this should eliminate many of the recurring disconnection problems with this terminal.
The Nano Routeur is configured as Access Point (and DHCP relay)
This structure has been up and running for 18 days , and everything is working fine 
! There have been no more disconnections of the EVCS from the access point, at all.
I hope this can help concretely some Victron customer having the same issue I had…
PS: I would recommend keeping the EV charger configured in DHCP, but with an IP address reservation on the DHCP server, so that IP address won’t change The benefit is that, if there is a problem, the charger will send a DHCP request and force the router to reallocate this address. This would not necessarily be the case with a hard-coded fixed IP address in the terminal. (Perhaps an excessive precaution.)
There are probably other AP that might fit, but I can testify that THIS ONE works well for me. 
Cerbo GX: v3.66, EVCS: v2.04