OK let’s make a summary.
This configuration partially works

config interface 'modem_1_1'
	option proto 'ncm'
	option ifname 'wwan0'
	option device '/dev/cdc-wdm0'
	option apn 'iliad'
	option dialnumber '*99#'
	option mode 'preferlte'
	# option delay '15'
	option metric '40'
	option disabled '0'

Router exits to Internet via modem but only if the modem is already connected to router before boot.
Ref: My modem doesn’t reconnect after it loses the connection
On router:

ping -c 4 8.8.8.8 ... OK
ping -c 4 google.com ... OK

Clients connected to router don’t exit to Internet but are connected to router
On clients:

ping -c 4 192.168.8.1 ... OK
ping -c 4 8.8.8.8 ... KO
ping -c 4 google.com ... KO

What’s there still to fix on the router? Bridge? Firewall?

Thanks

PS: I’ve none experience with networking on OpenWRT.