While trying to fix a problem with 5G connection on mine dropping out, I discovered that in Sweden on Telenor network the 5G on RUTX50 works only if mixed 4G/5G is enabled, then the NSA mode is active. If I switch to "pure" 5G, then SA mode is forced and well, 5G stops working stone cold. So excellent speeds under mixed 4G/5G, no connection under pure 5G. What mode are you using for Proximus 5G? Maybe try mixed mode?