You haven't really done anything wrong, you just haven't done enough. _ Take the group of ground points in and around IC5....they are all connected together but....is that group connected to the rest of the ground pioints on the board? _ No, it's isolated. _ Just make sure that ALL grounds are connected to every other ground in some way or another.....if you don't have a continuous connection between them all then you'll still have the ratline....and remember that ratlines are depicted as the shortest distance, and not necessarily the optimum or easiest route. _ We can't see the whole board so just as an example routing between the GND pad on P4 and the GND pad on R15 would connect everything under IC5. _ Regards.

I just found that, some of these things you can just close your browser (save your works first) and re-open it. This will do some tricks.

Does the autorouter report that any routes failed? If so, that explains the ratlines. You have a couple options if you want to continue using the autorouter: move components around until the ground path on IC5 is "visible" to other ground points on the board and hope it doesn't "hide" any other paths, or add a layer. The latter will pretty much force you into having the board professionally manufactured, but places like JLCPCB will make them less expensively than you probably could by the time you figure in the time and materials for doing it yourself.