I'm looking for a book from the '90s or maybe the early 2000s. The plot is as follows:
A ship is observed entering solar system. Another ship is sent to intercept and checks the derelict ship that is empty, having only a ring inside, similar to Stargate.
The ship lands somewhere in the North or South Pole and from the ship - the ring was a portal - a massive invasion starts. The aliens control enormous numbers of biological creatures (insectoids and so on) to conquer the Earth.
The story is of the main character (in the USA) that tries to arrive at a launch site to leave the Earth with a shuttle. It fails (the Earth is conquered) and goes to a secret location inside a mountain, where some sort of cryogenic installation may be used hoping that will escape in stasis.
The novel ends quite grimly... while entering in stasis pod a stone eating worm ener in location...
What I can tell is that the name of the author was of German origin and the novel isn't any of the ones proposed as answers to a similar question: "Novel about rendezvous with alien ship travelling through the solar system"