Frequencies and bands of all mobile network technologies:
https://www.sqimway.com
https://hicelltek.com/en/earfcn-calculator/
Shows the combination of bands of some smartphone models (Samsung, Apple, Huawei, etc.):
https://www.cacombos.com
https://www.halberdb.com
International cell map, unfortunately incomplete, but still a large crowd community:
https://www.cellmapper.net
Very good site about mobile communications
https://www.teltarif.de
BAKOM map (worse than Leutrim's masterpiece here, but still useful for certain situations, also to check the original every now and then)
https://www.funksender.ch or directly
https://map.geo.admin.ch/?topic=funksender&lang=de&bgLayer=ch.swisstopo.pixelkarte-farbe&layers=ch.bakom.radio-fernsehsender,ch.bakom.standorte-mobilfunkanlagen&layers_visibility=false,true&zoom=1&catalogNodes=403
Great site from Austria with mobile phone tests and a lot of technical information
https://www.notebookcheck.com
Very informative site with very in-depth analyses; inclusion of the geopolitical context, etc.) regarding mobile communications:
https://www.lightreading.com
NetMonster (Android application)
Displays frequencies well on Samsung, Huawei, BBK (Oppo, OnePlus, Vivo, Realme, iQoo), Xiaomi; good for most basebands (Samsung, Qualcomm, Huawei, Mediatek)
https://play.google.com/store/apps/details?id=cz.mroczis.netmonster&hl=de_CH
LTE Discovery
https://play.google.com/store/apps/details?id=net.simplyadvanced.ltediscovery&hl=de_CH