Can I use a wireless repeater or extender for 5G? or what is the best way to strengthen 5G signal?

Can I use a wireless repeater or extender for 5G? or what is the best way to strengthen 5G signal?

Out of curiosity: which operator gives you 5G access?

AFAIK (I haven't had hands-on opportunity to play with 5G, I'm just professionally working with 4G networks) you should stay away from any kind of playing with 5G signal. In 2G and 3G it was quite OK to use simple "signal boosters" to amplify signal. In 4G this is not fine any more, standardized solution is called lte relay which is sofisticated device. I expect that this trend will somehow extend towards true 5G ... although use of relay (or repeater) in modern high-speed mobile networks defeats one of basic cornerstones (giving high speed to users which is not possible in case of poor signal).