I am "cheritable", but only to a certain limit.
i.e. here https://community.teltonika-networks.com/59380/is-it-possible-to-trigger-script-on-failover
Customised firmware _MUST_ be better than the default stuff. Thus, it needs special knowledge and experience, to do it. However, to share it quite often is a two-sided sword. Because of freelance competitors, OR because of commercial users, not willing to spend some bucks at all.
As a real world example, I did lot of custom firmware for commercial hotspots, incl. Captive Portals, managed by "coova(-chilli)". Which is almost black magic to be configured correctly on LINUX, not to speak about openwrt. The projects I got were all from well known free lancer sites. Howver, a few times my bid was rejected, because too high. But several days later, I noticed very detailed questions regarding integration of coova into openwrt, and other project specific functionalities, on another forum.
Regarding our actual topic here in question, I do not consider it fair play, buying an off-the-shelf device for small money, and then asking for additional, special feature to be provided free-of-charge.
As a Teltonika customer, you either are an individual, asking for a reasonable priced device, you can live with, as provided, because for private use. I.e. for your mobile home.
Or you are a commercial client, needing a special feature, like the one we are talking about, for remote installs of a fleet of routers, i.e. in the area of IoT. But then you should be willing to pay a price for it.