I just moved to a new place and have a new internet connection (250Mbit over DSL with 35b supervectoring), and I would like to switch to OpenWRT for some of its features.
I am currently using a stock Fritz!Box 7530 as the Modem and Router, and I still have a Fritz!Box 7490 lying around. I'm currently thinking of installing OpenWRT on the 7490 (development seems to be going quite well) and keeping the 7530 on stock. I could then use the 7530 as the modem and connect the 7490 with OpenWRT as my router. However, I'm quite new to networking and honestly don't know if / how I would be able to use DECT / ISDN (which device would be responsible for that, the 7530 modem or 7490 router?). Furthermore, I've read contradicting opinions on whether it is possible to use the Fritz!Box as a standalone modem.
Can you give me some hints how I could set up my networking to take full advantage of the 250Mbit contract, have full DECT / ISDN functionality and use OpenWRT on my router? I'm also more than happy with links to documentation / resources that deal with similar topics, as I would love to understand this better myself. Anyway, I'm very thankful for any answers and directions you can give me.
Fritzbox 7490 is not supported yet, but work seems to have progressed quite a fair bit. Some people are currently running tests. See https://github.com/openwrt/openwrt/pull/5075. If the first post is still up to date, DECT / ISDN telephony is or will not be supported, but that is fine, as long as you opt to use this device simply as wifi repeater or dump AP.
Your plan to keep the 7530 on stock seems a good plan, if you want to make use of DECT / ISBN. If so, then they will only connect to the 7530, therefore the connection of your DECT devices is capped by distance and capabilities of the 7530 and the quality of your other DECT devices. The 7490 will be irrelevant in that regard.
I am less knowledgeable about Modems, so maybe somebody else can comment on that.
Thanks a lot for the answer, this is already very helpful. I think I'll have to play around with both devices for a bit to see how I can get ISDN / DECT to work, but it's good to know that my general plan should work
Keep one of them on stock (probably the 7530, as that one likely gets longer OEM firmware support) to deal with the phone stuff, in IPoE mode these can work fine as SIP pbx/ ATA and DECT base behind an OpenWrt router. There is no way to get DECT or the other phone features working on OpenWrt.
I would like to start by just setting up the DSL connection and basic routing, the phones aren't that important for now. Is there any guide how to set up OpenWRT with an external modem that would work for my case? I remember reading that you should reset the Fritz!Box you want to use as a modem (7530 in my case), disable everything during the setup and set it so that it doesn't try to log in but only forwards the connection so you can set up OpenWRT with the ISPs login data (with PPPoE or something like that?), but I simply can't find where I read that. Do I set one of the 7490s LAN ports to PPPoE and just enter my ISPs login information? What else needs to be configured for internet to be available on all other LAN ports and over WiFi?
Sort of, you remove that port fro br-lan, and use its device/interface name to configure as wan using PPPoE. But, on the danger of repeating myself, your FB 7490 will not be able to do interesting things at your link's 250/40 Mbps, it simply is too old a SoC, sorry.
Also fritzos does, as far as I know, not allow to easily configure modern fritzboxen into bridged-modems (but there likely are work arounds).
I already have the build system set up (since I had to compile the 7490 version myself anyways) - but before I go through the effort of installing OpenWRT on another router: when I compile following this guide you posted, will it have proper support for VDSL2+ (since it"s still in development as far as I can tell)?
I honestly can not tell you, I am still on a profile 17a VDSL2/Vectoring+G.INP link. Reading the linked thread makes me believe it works well enough to b worth testing, as I know that the 7490 is going to be too weak for a 250/40 link.
Personally, I'd get the "router" problem out of the way first - by getting something well supported by OpenWrt that is capable of doing 250/40 MBit/s (without sqm, ipq806x would do - mt7622 probably preferred, ipq807x should also do (as you don't seem to be adverse to building work-in-progress code)), then you can check how far you can get with the 7530 in mere modem duties (it's officially -explicitly- unsupported by AVM, but said to -kind of- work on a totally unconfigured device); the 7490 with stock firmware can be used for all your phone needs (I'm just a little worried about its future OEM support state in this capacity, that's why I was hinting at the 7530 instead (which is still being sold)).
Just for reference, my setup isn't (wasn't) that much different:
Draytek Vigor 130b as VDSL2 (vectoring, not super-vectoring, but the vigor 167 would be its modern super-vectoring capable counterpart) modem; I have switched ISPs to fibre now though (so no longer using any VDSL modem, but a Nokia G-010G-P ONT instead)
gateprotect GP-7543 (x86_64, Celeron J1900) running OpenWrt as router
(smart-)managed rtl8382m based switches running OpenWrt
various OpenWrt devices as APs
AVM Fritz!Box 7430 in IPoE mode, ,  (jailed into its own VLAN/ firewall zone) as SIP pbx, SIP ATA and DECT base
This has been working fine for me since 2017, originally on a 7362sl, now on the 7430 - as this one is going EOL (as well), I'll probably look into 7520/ 7530 or something similar in the future.
 "Anschluss an externes Modem oder Router", "Wählen Sie diesen Zugang, wenn die FRITZ!Box über "LAN 1" an ein bereits vorhandenes externes Modem oder einen Router angeschlossen ist."
 "IP-Client", "Diese FRITZ!Box ergänzt das Heimnetz als IP-Client und benutzt eine vorhandene Internet-Verbindung. Sie erhält vom Router eine IP-Adresse. Die FRITZ!Box wird Teil des vorhandenen Heimnetzes und übernimmt dessen IP-Adressbereich. Die Firewall der FRITZ!Box wird dabei deaktiviert."
 "Portweiterleitung des Internet-Routers für Internettelefonie aktiv halten", "Portweiterleitung aktiv halten alle: 30 Sekunden"
I've installed OpenWRT on my 7530 now, just to test it. However, I'm stuck on how to configure it. Since almost noone seems to have done this, I can't find any information on which DSL settings I could use. Are you able to help me out in that regard @moeller0?
There shouldn't be much to configure. If you included LuCI in your build, you should see the DSL status on the front page. If not, you need to run ubus call dsl metrics to show the state. You might get a sync with default settings, at least I get one on profile 17b.
I can log in via ssh and via http (LuCI), no problem. The installation worked and I can sometimes get the DSL to sync. However, it usually disconnects after a few seconds, and I would like to get 35b supervectoring to work, because otherwise it's capped at 100Mbit, right?
The dsl0 interface shows up, and by using dsl0.7 (so with VLAN-ID 7) I can sometimes get it to sync, but it disconnects after a few seconds. Can I somehow see (through the CLI or LuCI) which Annex / Tone etc. the connection is currently using? Because I always select multiples in the GUI.