Hello, it seems that an external antenna for the Arduino MKR Wifi 1010 is not feasible per the component specs (although you state that connector j10 is designated for an external antenna, btw from where did you get that advice?). This board uses a NINA-W102 module from uBlox. Take a look at the datasheet for this part located at: NINA-W10 series Stand-alone Multiradio modules Datasheet
From the datasheet:
The NINA-W102 module is equipped with a PIFA antenna. The RF signal is not connected to any
2.5.1 Internal antenna
Both NINA-W102 and NINA-W106 have internal antennas specifically designed and optimized for the
NINA module. The NINA-W102 module is equipped with a 2.4 GHz PIFA antenna and the NINA-W106
module has a 2.4 GHz PCB trace antenna...
In both NINA-W102 and NINA-W106 designs, keep a minimum clearance of 5 mm between the
antenna and the casing. Also, keep at least 10 mm of free space around the metal antenna including
the area directly below it. If a metal enclosure is required, use NINA-W101 and an external antenna.
And then section 4.2.7 contains detailed Wi-Fi radio characteristics. So it seems like a NINA-W101 would have allowed more flexibility.
Section 4.2.10 (Pp.25-26 for the NINA-W102) depicts the Antenna radiation patterns which might help you to optimally orient the board.