Indeed, all the passive PoE injectors seen on Teltonika product page is VAC, therefore you should look for a 3rd party one. One thing to note is voltage, RUT241 supports up to 30 volts, thus PoE injector’s output voltage should be 12 or 24 V. Here’s one found on amazon: https://www.amazon.com/Passive-PoE-Injector-24V-Compatible/dp/B0BWMQ7P5J
Assuming you can power the RUT241 device via LAN1 port ONLY using Passive PoE ONLY, and and mindfull of its maximum power consumption, then the Mikrotik RBGPOE-CON-HP PoE Converter (active to passive), should be more than enough for you.