Modem+OpenWrt: need to restart OpenWrt when modem powercycle

I have speedport entry2 in pure modem mode, and have Raspi4 with OpenWRT behind it (through a managed switch SG108PE, letting WAN an LAN run on the same port on Raspi4).
Unfortunately the jack of the power adapter of entry 2 is a little loose, I managed to power cycle it accidentally while I was trying to plug an Ethernet cable on the second port. I put the jack back immediately, but then the internet connection didn't come back. After several minutes of trying out (like restarting wan a few times), I restarted the whole Raspi4, then it did come back.

It looks like when the modem is restarted for some reason, the router has to be restarted as well. It causes extra worry. Like, power outage may happen for a short time, and if the modem and the router gets the power back in an wrong order, I will have to restart the router manually. But I might not be there, then the internet is out till I come home.

Is it always the case if you put any OpenWRT behind a pure modem? Or is it just entry2 (or raspi) ? Is it possible to set something so that it will not happen? I don't really understand the mechanism of this phenomenon.

I would appreciate if someone could give me advice !

Are you able to set static IP address for the RPi on the modem? If yes, how does it behave after that?

Well, entry2 is a pure modem, so it won't assign any IP address to RPi.

Nobody is replying.... many of you have an OpenWRT router behind a pure modem.... perhaps you could tell me if you also have to restart the OpenWRT router in case the modem gets power-cycled or restarted ?

No, I have tried both a BT HomeHub5A (running OpenWrt but configured as bridged-modem as far as the connected OpenWrt primary router is concerned) and a Zyxel VMG 1312 B30A and a reboot or disconnect of that modem device from power, DSL, or ethernet lines will not require rebooting the primary router.

But I always have the modem connected directly to the router so no switch in between.

Thank you for your reply ! I see, may be the switch makes the things complicated.... now I understood why nobody replied at first: nobody has this problem, and perhaps nobody connects Raspi-OpenWRT behind a modem through a managed switch: my Raspi has just one leg. (one port for WAN+LAN)
Just in case someone is very knowledgeable and can tell me why my system has this weird property, here is my situation in more details:

VLAN7 is WAN, 11, 15, 16 is for various LANs. On #2 there are APs.