What do you mean by the PC unable to reach 5Ghz? Do you mean the wireless signal?
If the PC is too far from the modem/router and you want to have a better performance/speed I would suggest to use ETHERNET cable instead.
Coz, 5ghz is useless if the PC is too far from the modem/router.
Read this https://community.tpg.com.au/t5/Broadband-Internet/2-4ghz-vs-5ghz/m-p/372#M114