There seem to be some problems with reception, antennas, modems, heat, battery life …
Could it be a workaround to use a Librem 5 without a SIM-Card and instead connect it via wifi tethering to a (probably already available) iOS- or Android-Phone?
You could carry on using your old phone for calls, sms and maybe WhatsApp etc. and activate tethering for the Librem 5 to use. – At least for a transitional period.
On the Librem 5 you could do any other (privacy, security and freedom critical) things. Maybe via a VPN.
What problems would that way of usage solve and what problems would it cause?