Found Skaisták. It is located further towards Čaka iela. No sign with working hours. Probably it was closed with rolls down last time, otherwise I have no idea how I missed it.

From the corner of Matīsa iela 51/53 building toward the Čaka iela: first is Essieviete (hairdresser?), then veterinary pharmacy Terion, then just entrance to the building and finally Skaisták. After that is only passage to the yard, some place without markings and passage to the yard of Matīsa 49  building.

Attached photo(s):
https://streetcomplete.app/p/259094.jpg
https://streetcomplete.app/p/259095.jpg