EU_868 or local conventions: the ISM band depends on the country. A 915 antenna underperforms at 868, and the hardware must be the right variant.
An ISM band is not universal. The legal frequency changes with the country, and a wrong choice yields a weak or unlawful link.
The band follows the country
In Europe, the useful sub-GHz band sits around 868 MHz, with the EU_868 plan. Elsewhere, local regional conventions impose other plans and other duty-cycle limits.
Loading the wrong regional plan into firmware means transmitting on an unauthorized frequency or with a forbidden duty cycle. The link may work on the bench and remain illegal in service.
The hardware must follow
An antenna is tuned to a center frequency. A 915 MHz antenna used at 868 MHz shows a degraded return loss. Radiated power drops and range collapses, with no visible error.
The RF front-end must be the right band variant. A wideband module covers a useful range but stays optimized around a center. Pick the target-country variant, tune the antenna to that band, then load the matching regional plan. In that order.