To avoid startrails you must track the stars. With telescopes and DSLRs you need more or less large telescope mounts.
But with phones and the app DeepSkyCamera
you need only a lightweight portable mount and a tripod. Thats it. The following picture shows two phones (LG G6 und Huawei P9) on two portable mounts
(iOptron SkyTracker and AstroTrac).
The portable mount must be leveled and must point to the north pole (when you are on the northern hemisphere) or to the south pole
(when you are on the southern hemisphere)
of the sky. The polarfinder is mostly included. So you can easily find Polaris and the
north pole or the south pole in constellation Octans. The mount tracks the stars.