I recommend typing your postcode into the Three Network Status checker to see if there is any work or known problems in the area. Does this happen all the time or only at certain times?
If nothing shows up on the checker is the router in the optimal place for the signal, on the ZTE you have a location checker. I don't have the Zyxel hub, however, I think there is an app that you can use to help find the optimal location. If you have the Zyxel then there is a useful guide here LTE / NR 5G Issues Zyxel vs. Provider Issues – Zyxel Support Campus EMEA that may help you troubleshoot, You also need to confirm with Three technical support if you are running the latest firmware, this can effect the connection.
On the ZTE hub there isn't much info on troubleshooting except to reset to the router (hole underneath), use a sim card tool in the hole underneath for a couple seconds. You will know if you have done it correctly as the lights will go off and come back on (use the details on the label underneath the router to log back in again after resetting and set up everything again). They also suggest to take out and re-insert the SIM card as well (underneath) to see if that works.
For the Zyxel NR5103E just type the model number in a web search and you will see the community and they also have a support campus for troubleshooting, however, its best to speak to Three tech support if you're not technically minded.
