Lantiq hardware should be supported, it's actually one of the few (if not the only) DSL hardware supported on OpenWrt. I know it's for the LTE version, but the DSL version seems to be documented as well here: Support for Easybox 904 LTE (along with links to the old, dead forum and whatnot). Hopefully you can get some information from there.
The Easybox 904 xDSL should work in the various (annex B) ADSL modi as any other lantiq device (I've only tested VDSL2 on it myself). VPI/ VCI settings and VLAN tagging (if necessary) need to be configured as needed.
nano isn't part of OpenWrt's default (preinstalled) package set (but you can install it), (busybox-) vi however is.
vi isn't the most beginner friendly text editor, but it's small and expected to be present on every Linux/ UN*X system, so some basic familiarity for simple usage is good to train. If you are familiar with it, it can be a very mighty text editor, able to deal with very complex tasks (even if it might look frightening at first) and huge files.
busybox vi has a limited feature set compared to the one described in those guides (which are meant for the full command implementations of e.g. ex-vi, nvi, vim, etc.), but the basics for editing/ inserting ([ESC][i]) and saving/ closing ([ESC]:wq) a text file are the same.
First of all, are you sure that you need ADSL annex B (or annex J), which is rather uncommon outside of germany (and parts of Austria/ Switzerland)?
If you require annex A, the Easybox 904 xDSL is not going to help you.
In general, the PPPoE variant of https://openwrt.org/docs/guide-user/network/wan/isp-configurations#adsl1 with option vci '35' and option ifname 'dsl0.510' should work, except that the Easybox 904 xDSL hardware doesn't support annex A, so it would only work (if supported by your ISP) with option annex 'a' or option annex 'j' - and you may have to experiment with the option tone settings.
Unfotunly i do not know when the Modem settings are really in use, when i have an equal Problem i have reboot the whole Router after each change.
If someone have the answer of the question when are the modem settings are really in use please wrote here.
If you are a Member of an Annex-B Country try this:
ADSL Annex B