This book wasn't as good as the first.
Plot-wise, there's nothing wrong, per se, with the premise in and of itself... but the first book didn't exactly need a sequel either. The spot it left off at was perfectly fine: the main conflict had been resolved, permanently; the last of the trolls had been driven out, and it was well and truly established that they weren't coming back. It could have worked perfectly well as a standalone if the authors had left it at that.
That just might account for a large chunk of the problem, and it might just explain why this story frequently reads more like a fanfic than it does a true sequel. The conflict that drove the previous book is no more, so if they want to be able to keep this train going, the authors are either going to have to retcon it back into existence, or pull a new one out of thin air. The latter is definitely preferable to the former, but... it still is difficult, sometimes, not to wonder what exactly the point is.
The story's other big weakness is the fact that it drops Isolfr as a POV character, a choice behind which it's frankly difficult to see the reasoning. Isolfr was an interesting character, and he had a compelling arc: as someone who took a path in life he never would have chosen on his own initiative because he saw it as his duty; as a straight man who took up a position where he knew he might very well end up having sex with a whole lot of men, again out of a sense of duty; as (a personal favorite) a fish out of water trying to navigate a world he'd previously only known through a lot of exceedingly unflattering rumors; as (another favorite) someone whose moral sense was beginning to clash strongly with his sense of duty to his people. Then, too, there was the complex web of relationships the previous book built up, with Isolfr as the central thread holding that web together: with his family, with his friends, with his daughter, with his svartalf allies, with the various men vying for his attention, and, of course, with his bonded wolf. Without Isolfr's compelling viewpoint, the whole thing promptly comes unraveled, and all we're left with is a barely-connected collection of threads blowing aimlessly in the wind.
In terms of workability, the premise is fine. In terms of technical competence, the writing is also fine. There's just not much to get emotionally invested in, and this is the sort of story that needs emotional investment in order to be enjoyable. The first book was dark, it was gritty, it was brutal, it was morally gray, it ran the gamut from hot to horrifying over the course of a single page, and it was a damn good read. There was a really strong air of focusing on what was important not in spite of the fact that the world was ending, but because of it. This... while there is some good stuff there in Skjaldwulf's relationship with Otter and in Vethulf's efforts to grow into his role as a leader, overall, it's a lot more aimless. It feels recycled, as if the authors, after writing themselves into a corner where their heroes no longer had any trolls to fight, decided to throw some Romans at them instead, but with very little rationale behind that decision beyond "Hey, let's publish more books so we can make more money!"
Then there's the fact that, unlike the first book, this one doesn't have any sort of real arc or tell any sort of self-contained story. There's no direction and no climax. Instead, the whole thing from beginning to end serves as little more than a setup for the final book in the series. Again, setting up stuff for subsequent books is fine, that's what a series is... but even a middle book ought to have some sort of resolution, or closure, or character arc, something to make this a story worth reading in its own right as opposed to just being a trailer for the grand finale. No one's expecting you to topple the Empire before the final book, but you could at least take out a Death Star or split the Fellowship.