Network Repeater Installation for Smart Meters

400Mhz to 4GHz is not too wide for one antenna although once the application is known you can choose a suitably sized antenna that can have a higher gain. Bandwith and gain are trade-offs.

42GHz, if it's ever used, is for Pico and Femto cells which are extremely short range as they suffer very badly from atmospheric water absorption issues for anything longer range. They are used to increase capacity at the expense of range. The six frequencies 800MHz to 2.6GHz  are used for anything with a longer range.

The solution ​@martynjgreen mentions sounds like an extension of the Heath Robinson DIY setup I’ve often suggested to customers in this sort of situation in the Northern CSP area (i.e. N of ~Manchester).  It’s pretty basic: a 2 x 710mm dipole high up on the wall facing the transmitter and a 1/8 one - a bent wire coat-hanger, say - taped to the CH, with as short a run as possible of low-loss coax cable to join them. If there’s any appreciable distance between the little aerial and the CH, the signal will probably not be strong enough, so a booster would be needed to make it available to all the meters. That’s where a radio engineer needs to take over. 

​@Firedog has given a simple but often effective solution. The passive relay. The external antenna, bathed in ample radiation from it's relatively lofty altitude sends the signals down the coax to a simple antenna in the radiowave deficient cellar. There is almost nothing to be lost apart from the time to drill a hole into the cellar which would have to be done for the CH. It's unconventional but it has worked in the past. That was what ​@Nukecad mentioned earlier and possibly lost in a side tributary of nested comments which is why I don't like nested comments.