Late model Garmin devices apparently depend on a DEM to supply "corrections" to elevation data. Without the corrections elevation data from my VivoActive 3 are useless. Regreetably they are worse than useless if I accept Garmin's offer (at CarminConnect) to correct them.
Since the only way Garmin could know anything about the elevation of points on the Earth's surface (and because of a specific mention of DEMs in another context) I assume that this function uses a digital elevation model as its source of information. In the case of Iceland (where I live) I wouldn't be surprised if they are using the old GTOPO30 data set, Iceland bering well north of the Shuttle SRTM mission coverage area. This data set is worse than useless.
Recently the local Geodetic Institute published a reasonably accurate DEM, (https://atlas.lmi.is/mapview/?application=DEM) which I believe is available for use at no cost. Could you PLEASE use either this model or the equally accessible international DEM data that have been recently published to improve the performance of your products in this country?