I finally found the time to test it and got it working! Sitting down and doing an exact 1:1 comparison with the Taiwan SIM card on the FP3+ did not do it. But I did another 1:1 comparison with my German SIM card and that helped in the end. It seems like I have to add an entry ia for the APN type.
For completeness:
I also saw that in the related thread you found a solution. I put it here in case someone finds my thread first and this does not help.
If I use the German SIM card on the FP5 the APN type setting has two more entries: mms and ia. MMS should not help here but I thought I mention it because in the other thread people were commenting on MMS in Switzerland as well.
So if I add ia to the APN type IMS registration works (according to *#*#4636#*#*). I will ask someone from work to give me a call next week to make sure this also works.
In hindsight, I could have probably gotten there faster. Maybe I did not look carefully enough. Interestingly, ia does not seem to be necessary on the FP3+. I have a screen shot of the APN settings and it does not show up there.
For completeness here are the APN settings with the Taiwan (currently working version left; initial, broken version right) and German carrier side by side (- indicates no entry):
| Field | Taiwan SIM (working) | German SIM | Taiwan SIM (broken) | |||
|---|---|---|---|---|---|---|
| Name | 中華電信 (emome) | IMS | Telekom Internet | IMS | 中華電信 (emome) | IMS |
| APN | emome | ims | internet.telekom | ims | emome | ims |
| Username | - | - | telekom | - | - | - |
| Password | - | - | **** | - | - | - |
| APN type | default, supl, ia | ims | default, mms, supl, ia | ims | default, supl | ims |
| MMSC | - | - | some t-mobile.de url | - | - | - |
| MMS proxy | - | - | some IPv4 address | - | - | - |
| MMS port | - | - | 8008 | - | - | - |
| Authentification type | - | - | PAP | - | - | - |
| APN protocol | IPv4/IPv6 | IPv4/IPv6 | IPv4/IPv6 | IPv6 | IPv4 | IPv6 |
| APN roaming protocol | IPv4 | IPv4/IPv6 | IPv4 | IPv4/IPv6 | IPv4 | IPv4/IPv6 |
| Network type | unspecified | LTE, & a long list | LTE, & a long list | LTE, & a long list | unspecified | LTE, & a long list |