There's no maximum length of cable, however please note that the longer the cable, the higher the attenuation, so if you want to use external antenna you'll probably need it with better dBi gain than the standard one.
We suggest to ask your local supplier/expert for possibilities. There're two requirements:
1) your antenna needs to support UMTS/LTE wavebands used by your carrier
2) it needs to have SMA (m) connector.