I’m familiar with this situation and used to use ibooost booster from motorhomewi company. Very good BUT the problem was not only low signal strength but low server capacity and no booster will overcome this. Worse still, one internet company which served the site near Mojacar sought to minimise complaints by tweaking their software such the it would only connect to a single device eg laptop and not to any form of booster ( many of which act as hotspots and allow multiple connections ) This stopped the habit of many useres connecting via a booster and then leaving the connection on 24/24 with Skype or a VPN running all the time.
Also boosters need something approching line of sight to the local transmitter, which was becoming ( on our site) increasingly difficult to obtain from caravan level even with a mast due to increasing numbers of large tall motorhomes.
This last trip I bought a wifi hotspot device from TPLink and got a monthly contract from M3 unlimited at about £25/month which roamed happily in Europe with speeds of around 10 down 2 up. The device is SIM free and so a local data SIM can be bought and fitted. Multiple devices can be used. The only snag is that some cards need to be initiated in UK and that EU limit the amount you can supposedly use. Allegedly 14 Gb in 3 months except 19 in Spain. This presumably does not apply to a local SIzM. Local SIM looks like the answer to the possibly high roaming charges after we completely leave EU. The TP link device is smaller than a phone and charges by USB cable as usual. Battery life is claimed around 5 hours. Works well in the car too.
So all in all suggest you look at this before buying booster