No cellular network after OTA to lollipop (Nexus 4)

I have a Nexus 4 with Kitkat, working fine with t-mobile (USA). Yesterday, I did an OTA to Lollipop (LRX21T; baseband M9615A-CEFWMAZM-2.01701.04). But after that the cellular network only briefly works. First problem, I encountered was the lack of data connection, but after I changed the APN setting, it worked...but only briefly. Every time I reboot the phone, it can make phone calls, and access the data network for a few minutes (or sometimes I have no access to begin with). But then the network indicator becomes either an 'empty triangle' or a 'full triangle with an exclamation point'. Then, I can't make phone calls or use the data afterwards. Very strangely, I just found out that if I connect to wifi, I can make phone calls suddenly. If I turn off wifi, I can't make phone calls any more. What is going on? Does anybody have the same problem? Does anybody a known workaround? My current APN setting is: T-Mobile GPRS APN: fast.t-mobile.come Proxy: not set Port: not set Username: not set Password: not set Server: not set MMSC: http://mms.msg.eng.t-mobile.com/mms MMS proxy: not set MMS port: not set MMC: 310 MNC: 260 Authentication type: not set APN type: default,supl,mms,dun APN protocol: IPv4 APN roaming protocol: IPv4 Bearer: unspecified MVNO type: none MVNO value: not set Further testing using *#*#4636#*#* * GSM service: in service * GPRS service: disconnected * Network type: UMTS * Call redirect: false * Call status: idle * radio resets: 0 * data attempts : 0 * data successes: 0 * cellinfolistrate 2147483647 * IMS registration required is 'off' * SMS over IMS is 'off' * SMSC: _______ [update][refresh] * Toggle DNS check: 0.0.0.0 not allowed **EDIT**: During the moments when the network works, **GPRS service** is 'connected'. **EDIT2**: If I force using 2G, it works fine. Just with unbearably slow speed.