OpenWrt on a modem router, or two routers, or modem+router with VoIP?

[doremifajb]

Thank you very much for your info !
I'm not sure if I understood you right, you wrote that I would need three devices, but if I choose VDSL modem to be a fritzbox router, then I would just need two devices ? (but then I will have to find a right one so that I can avoid double NAT, if it's desirable that way)

And three devices would be needed if I do:
Pure Modem -- OpenWRT -- low-end FB for Phone (perhaps in IP client mode?) or DECT basis.

And for the choice of OpenWRT router, I guess I can follow what you wrote me before:

Capacity of routers: where/what to look?

So to summarize the situation a bit. You have a AVM FRITZ!Box 4020 on a 100/40 MBit/s line 750 MHz QCA9561 (single core mips 74Kc) 16 MB flash 128 MB RAM 2.4-GHz-only 802.11n wireless, which you're not going to use (for the bulk of your needs) anyways, offloading the wireless side to dedicated APs up to ~35 concurrent devices in a commercial/ hospitality setting As you have correctly noted, apart from the 'unused' wireless side[0], these are exactly the same specifications as for the Archer…

right ?

For the second option you are suggesting that I would install OpenWRT on, e.g. FB7360, that comes with VDSL2 100 capability, and do it with one router, is that right ? Isn't it rather difficult (for an inexperienced person) to connect VoIP to it ?