Init.d script dependency

I created a small init.d script that initializes the modem via the AT interface.

I named it S21_modem_init, it does run at the designated point in time (after S20network), my only problem is that pppd starts earlier then this script is actually finished. This only runs at boot(), so no state handling like service_start() or anything like that is introduced.

So the question is, how can I make an Init.d script depend or wait for on another Init.d script to finish? Atm I use a PPPD parameter to wait a fixed 10 seconds, but that is not going to be a long term solution once this is switched to QMI or MM.

no access to a openwrt device currently, but is pppd another init.d script ?
if so, disable the auto run in init.d, and start if from within your own script ?

I think it is netifd which controls these parts, S20network just invokes /sbin/netifd and thats how networking starts, as much as I understand it. My confusion is that normally init.d can be configured with dependencies on a "full" linux, but that seems not to apply here.

MOD: it is netifd, and it calls /lib/netifd/ppp-up and ppp-down.