Both versions are grammatically correct. Your version (with the adverb at the front) would probably be more common in most writing or speech — it is slightly unusual for an adverb to get separated from its verb phrase by another adverbial phrase like this. In this case it seems OK (at least to my ear); but I’d agree that in some cases the separated version can really be quite horrible, to the point of being ungrammatical; compare:
Sally ate the cake quickly with great pleasure.
?Sally ate the cake with great pleasure quickly.
However, in advertising (like in headline-writing), writers are trying to fulfil a lot of other demands in a very small space, so somewhat unusual phrasings often occur. In this case, putting “Get a high-speed roaming connection” first makes the subject of the ad more immediately clear, and putting “instantly” last gives the phrase a good punchy ending. I’d guess these are the sorts of things the writer had in mind.