There could be several reasons for this:
First you can try rebooting your phone near the Coverage Unit so it takes a fresh look at the available channels. Also verify that your phone is compatible with the channels that your CEL-FI model is boosting (maybe your handset is “unlocked” and actually does not fully support all your current Operator’s channels). iPhones can also show fewer “bars” of signal if the network is heavily loaded (learn more).
Note that CEL-FI RS1 and RS2 models do not support LTE. If your phone is LTE capable and therefore is not showing boosted service, if needed it is designed to switch over to non-LTE services when it needs to, such as in a call. This is true with or without CEL-FI and what matters is that now you have reliable service where you need it!
If you would like the benefits of LTE service as well, you can always upgrade your CEL-FI to a newer version that also supports LTE.