Ok--Cells on Earth-based life are deformed spheres (genus 0, i.e. no "holes" like a donut). That seems reasonable, as I figure it is easier for a spherical cell to form randomly than anything more complex. A spherical cell might have evolved when, for some reason, self-replicating molecules got other molecules attached to them to protect them.
Still, it's certainly imaginable that by some random chance, a toroidal cell developed--nuclear material in a ring shape, with an inner-tube-like membrane surrounding it. It is not necessary that the DNA or equivalent actually be a ring--it could still be a thread with ends unconnected. The only thing is, would such a cell have any kind of survival advantage in some alien environment? If so, there would be a lot more and they would evolve.
There is more than one way you could split a donut--they might reproduce by becoming first, a two-holed figure-eight shaped donut, then breaking apart into two donuts, or perhaps it is simpler if a ridge forms on the equator and on the inner equator and the two ridges then squeeze together, so the final result looks like a donut sitting on top of a donut, both horizontal, and then they split into two.
Now, at some point, by mutation and/or accident, some donut will split in such a way that the two resulting donuts are linked. If this were to provide a survival advantage, it would be the start of a multi-cellular life form. Two different donut species getting linked somehow (I'm not sure how, but I'm sure it could happen) could become symbiosis.
Fast forward several billion years....multicellular macroscopic life forms with toroidal cells linked up like chains, mesh armor, or the like. If there is a survival advantage for toroidal cells, the aliens might have trouble imagining an alien species could possibly survive with spheroidal cells--with no way for cells to link together, they would just fall apart into dust, wouldn't they?
Now, if I could figure out how a cell could have a Mobius strip membrane so it mixes its outside with its inside....not so likely I guess.